Merge from origin/emacs-27

45a64c97c7 (origin/emacs-27) Clarify semantics of trace-function CONT...
821760fdc4 Don't let a code literal get modified in mml parsing (Bug#...
74a92be16d * lisp/simple.el (kill-ring-save): Doc fix.  (Bug#40797)
3d0e859692 Minor doc clarification regarding fringe bitmaps
4d86c7f822 Fix documentation of fringe bitmaps
a76af88dd8 Tweak mutability doc a bit more
f7e488d206 Calc: fix autoload errors (bug#40800)
369761b36d ; * src/xdisp.c: Improve the introductory commentary.
a92ca1f177 Improve indexing of ELisp manual
5a25d17760 * lisp/image-mode.el (image-transform-resize): Remove FIXM...
37ebec3a95 Improve the default value of 'doc-view-ghostscript-program'.
ba6104d1e8 Change doc-view-mode-map prefix key 's' to 'c'.
400ff5cd19 Improve wording about constants
d2836fe71b Improve the default value of 'doc-view-ghostscript-program'.
fc55f65305 Minor improvements in documentation of the last change
a64da75961 Add image-auto-resize defcustoms to image-mode.el
692ad40539 Improve the documentation of tab-bar and tab-line

# Conflicts:
#	etc/NEWS

22 files changed, 436 insertions, 205 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index cbc00ee4eef..536f4cb5daa 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -735,6 +735,7 @@ in a way that does not interact well with @code{highlight}.
 @cindex @code{tab-line} face
 Similar to @code{mode-line} for a window's tab line, which appears
 at the top of a window with tabs representing window buffers.
+@xref{Tab Line}.
 @item vertical-border
 @cindex @code{vertical-border} face
 This face is used for the vertical divider between windows on text
diff --git a/doc/emacs/emacs.texi b/doc/emacs/emacs.texi
index 60f2be9a513..6b82aeb8234 100644
--- a/doc/emacs/emacs.texi
+++ b/doc/emacs/emacs.texi
@@ -517,6 +517,7 @@ Multiple Windows
 * Displaying Buffers::  How Emacs picks a window for displaying a buffer.
 * Temporary Displays::  Displaying non-editable buffers.
 * Window Convenience::  Convenience functions for window handling.
+* Tab Line::            Window tab line.
 
 Displaying a Buffer in a Window
 
diff --git a/doc/emacs/files.texi b/doc/emacs/files.texi
index 7d57555ce33..56ce7fdea19 100644
--- a/doc/emacs/files.texi
+++ b/doc/emacs/files.texi
@@ -2113,8 +2113,6 @@ point.  Partial Completion mode offers other features extending
 
 @findex image-mode
 @findex image-toggle-display
-@findex image-next-file
-@findex image-previous-file
 @cindex images, viewing
   Visiting image files automatically selects Image mode.  In this
 major mode, you can type @kbd{C-c C-c} (@code{image-toggle-display})
@@ -2122,12 +2120,34 @@ to toggle between displaying the file as an image in the Emacs buffer,
 and displaying its underlying text (or raw byte) representation.
 Additionally you can type @kbd{C-c C-x} (@code{image-toggle-hex-display})
 to toggle between displaying the file as an image in the Emacs buffer,
-and displaying it in hex representation.
-Displaying the file as an image works only if Emacs is compiled with
-support for displaying such images.  If the displayed image is wider
-or taller than the frame, the usual point motion keys (@kbd{C-f},
-@kbd{C-p}, and so forth) cause different parts of the image to be
-displayed.  You can press @kbd{n} (@code{image-next-file}) and @kbd{p}
+and displaying it in hex representation.  Displaying the file as an
+image works only if Emacs is compiled with support for displaying
+such images.
+
+@vindex image-auto-resize
+@vindex image-auto-resize-on-window-resize
+If the displayed image is wider or taller than the window in which it
+is displayed, the usual point motion keys (@kbd{C-f}, @kbd{C-p}, and
+so forth) cause different parts of the image to be displayed.
+However, by default images are resized automatically to fit the
+window, so this is only necessary if you customize the default
+behavior by using the options @code{image-auto-resize} and
+@code{image-auto-resize-on-window-resize}.
+
+@findex image-transform-fit-both
+@findex image-transform-set-scale
+@findex image-transform-reset
+To resize the image manually you can use the command
+@code{image-transform-fit-both} bound to @kbd{s b}
+that fits the image to both the window height and width.
+To scale the image specifying a scale factor, use the command
+@code{image-transform-set-scale} bound to @kbd{s s}.
+To reset all transformations to the initial state, use
+@code{image-transform-reset} bound to @kbd{s 0}.
+
+@findex image-next-file
+@findex image-previous-file
+You can press @kbd{n} (@code{image-next-file}) and @kbd{p}
 (@code{image-previous-file}) to visit the next image file and the
 previous image file in the same directory, respectively.
 
@@ -2195,7 +2215,6 @@ can be used to transform the image in question to @acronym{PNG} before
 displaying.  GraphicsMagick, ImageMagick and @command{ffmpeg} are
 currently supported for image conversions.
 
-
 @findex thumbs-mode
 @cindex mode, Thumbs
   The Image-Dired package can also be used to view images as
diff --git a/doc/emacs/frames.texi b/doc/emacs/frames.texi
index 94218424d37..d9373b8bc78 100644
--- a/doc/emacs/frames.texi
+++ b/doc/emacs/frames.texi
@@ -1262,6 +1262,12 @@ sessions (@pxref{Saving Emacs Sessions}), the tabs from the Tab Bar are
 recorded in the desktop file, together with their associated window
 configurations, and will be available after restoring the session.
 
+Note that the Tab Bar is different from the Tab Line (@pxref{Tab Line}).
+Whereas tabs on the Tab Line at the top of each window are used to
+switch between buffers, tabs on the Tab Bar at the top of each frame
+are used to switch between window configurations containing several
+windows.
+
 @findex tab-bar-mode
   To toggle the use of tab bars, type @kbd{M-x tab-bar-mode}.  This
 command applies to all frames, including frames yet to be created.  To
@@ -1275,9 +1281,11 @@ is turned on automatically.  If the value is @code{t}, then
 tabs.  The value @code{1} hides the tab bar when it has only one tab,
 and shows it again when more tabs are created.  The value @code{nil}
 always keeps the tab bar hidden; in this case it's still possible to
-use persistent named window configurations without using the tab bar
-by typing the related commands: @kbd{M-x tab-new}, @kbd{M-x tab-next},
-@kbd{M-x tab-close}, @kbd{M-x tab-switcher}, etc.
+switch between named window configurations without the tab bar by
+using @kbd{M-x tab-next}, @kbd{M-x tab-switcher}, and other commands
+that provide completion on tab names.  Also it's possible to create
+and close tabs without the tab bar by using commands @kbd{M-x
+tab-new}, @kbd{M-x tab-close}, etc.
 
 @kindex C-x t
   The prefix key @kbd{C-x t} is analogous to @kbd{C-x 5}.
@@ -1286,7 +1294,8 @@ Whereas each @kbd{C-x 5} command pops up a buffer in a different frame
 tab with a different window configuration in the selected frame.
 
   The various @kbd{C-x t} commands differ in how they find or create the
-buffer to select:
+buffer to select.  The following commands can be used to select a buffer
+in a new tab:
 
 @table @kbd
 @item C-x t 2
@@ -1295,19 +1304,18 @@ buffer to select:
 Add a new tab (@code{tab-new}).  You can control the choice of the
 buffer displayed in a new tab by customizing the variable
 @code{tab-bar-new-tab-choice}.
+
 @item C-x t b @var{bufname} @key{RET}
 Select buffer @var{bufname} in another tab.  This runs
 @code{switch-to-buffer-other-tab}.
+
 @item C-x t f @var{filename} @key{RET}
 Visit file @var{filename} and select its buffer in another tab.  This
 runs @code{find-file-other-tab}.  @xref{Visiting}.
+
 @item C-x t d @var{directory} @key{RET}
 Select a Dired buffer for directory @var{directory} in another tab.
 This runs @code{dired-other-tab}.  @xref{Dired}.
-@item C-x t r @var{tabname} @key{RET}
-Renames the current tab to @var{tabname}.  You can control the
-programmatic name given to a tab by default by customizing the
-variable @code{tab-bar-tab-name-function}.
 @end table
 
 @vindex tab-bar-new-tab-choice
@@ -1316,7 +1324,7 @@ current before calling the command that adds a new tab.
 To start a new tab with other buffers, customize the variable
 @code{tab-bar-new-tab-choice}.
 
-  The following commands are used to delete and operate on tabs:
+  The following commands can be used to delete tabs:
 
 @table @kbd
 @item C-x t 0
@@ -1325,19 +1333,45 @@ To start a new tab with other buffers, customize the variable
 Close the selected tab (@code{tab-close}).  It has no effect if there
 is only one tab.
 
+@item C-x t 1
+@kindex C-x t 1
+@findex tab-close-other
+Close all tabs on the selected frame, except the selected one.
+@end table
+
+  The following commands can be used to switch between tabs:
+
+@table @kbd
 @item C-x t o
+@itemx C-@key{TAB}
 @kindex C-x t o
 @kindex C-TAB
 @findex tab-next
-Switch to another tab.  If you repeat this command, it cycles through
+Switch to the next tab.  If you repeat this command, it cycles through
 all the tabs on the selected frame.  With a positive numeric argument
 N, it switches to the next Nth tab; with a negative argument −N, it
 switches back to the previous Nth tab.
 
-@item C-x t 1
-@kindex C-x t 1
-@findex tab-close-other
-Close all tabs on the selected frame, except the selected one.
+@item S-C-@key{TAB}
+@kindex S-C-TAB
+@findex tab-previous
+Switch to the previous tab.  With a positive numeric argument N, it
+switches to the previous Nth tab; with a negative argument −N, it
+switches back to the next Nth tab.
+@end table
+
+  The following commands can be used to operate on tabs:
+
+@table @kbd
+@item C-x t r @var{tabname} @key{RET}
+Rename the current tab to @var{tabname}.  You can control the
+programmatic name given to a tab by default by customizing the
+variable @code{tab-bar-tab-name-function}.
+
+@item C-x t m
+Move the current tab N positions to the right with a positive numeric
+argument N.  With a negative argument −N, it moves the current tab
+N positions to the left.
 @end table
 
 @node Dialog Boxes
diff --git a/doc/emacs/glossary.texi b/doc/emacs/glossary.texi
index 416431b8cd4..4d622ec0e3f 100644
--- a/doc/emacs/glossary.texi
+++ b/doc/emacs/glossary.texi
@@ -1367,7 +1367,7 @@ configurations.  @xref{Tab Bars}.
 
 @item Tab Line
 The tab line is a line of tabs at the top of an Emacs window.
-Clicking on one of these tabs switches window buffers.
+Clicking on one of these tabs switches window buffers.  @xref{Tab Line}.
 
 @anchor{Glossary---Tags Table}
 @item Tags Table
diff --git a/doc/emacs/misc.texi b/doc/emacs/misc.texi
index a15aa17a66f..47f195d0b20 100644
--- a/doc/emacs/misc.texi
+++ b/doc/emacs/misc.texi
@@ -617,12 +617,12 @@ of pages to display.  A slice is a rectangle within the page area;
 once you specify a slice in DocView, it applies to whichever page you
 look at.
 
-  To specify the slice numerically, type @kbd{s s}
+  To specify the slice numerically, type @kbd{c s}
 (@code{doc-view-set-slice}); then enter the top left pixel position
 and the slice's width and height.
 @c ??? how does this work?
 
-  A more convenient graphical way to specify the slice is with @kbd{s
+  A more convenient graphical way to specify the slice is with @kbd{c
 m} (@code{doc-view-set-slice-using-mouse}), where you use the mouse to
 select the slice.  Simply press and hold the left mouse button at the
 upper-left corner of the region you want to have in the slice, then
@@ -631,10 +631,10 @@ button.
 
   The most convenient way is to set the optimal slice by using
 BoundingBox information automatically determined from the document by
-typing @kbd{s b} (@code{doc-view-set-slice-from-bounding-box}).
+typing @kbd{c b} (@code{doc-view-set-slice-from-bounding-box}).
 
 @findex doc-view-reset-slice
-  To cancel the selected slice, type @kbd{s r}
+  To cancel the selected slice, type @kbd{c r}
 (@code{doc-view-reset-slice}).  Then DocView shows the entire page
 including its entire margins.
 
@@ -791,7 +791,7 @@ the same number of columns as provided by the shell.
 @vindex shell-command-prompt-show-cwd
   To make the above commands show the current directory in their
 prompts, customize the variable @code{shell-command-prompt-show-cwd}
-to a non-nil value.
+to a non-@code{nil} value.
 
 @kindex M-|
 @findex shell-command-on-region
diff --git a/doc/emacs/modes.texi b/doc/emacs/modes.texi
index 92d60d2d7c2..f5fb6b1e797 100644
--- a/doc/emacs/modes.texi
+++ b/doc/emacs/modes.texi
@@ -299,7 +299,7 @@ Bars}.
 Tab Bar mode gives each frame a tab bar.  @xref{Tab Bars}.
 
 @item
-Tab Line mode gives each window a tab line.
+Tab Line mode gives each window a tab line.  @xref{Tab Line}.
 
 @item
 Transient Mark mode highlights the region, and makes many Emacs
diff --git a/doc/emacs/windows.texi b/doc/emacs/windows.texi
index 910ef8fd88d..cb5e9bce4d1 100644
--- a/doc/emacs/windows.texi
+++ b/doc/emacs/windows.texi
@@ -21,6 +21,7 @@ one frame.
 * Change Window::       Deleting windows and changing their sizes.
 * Displaying Buffers::  How Emacs picks a window for displaying a buffer.
 * Window Convenience::  Convenience functions for window handling.
+* Tab Line::            Window tab line.
 @end menu
 
 @node Basic Window
@@ -542,16 +543,6 @@ Reference Manual}), and cannot exceed the size of the containing frame.
 @node Window Convenience
 @section Convenience Features for Window Handling
 
-@findex global-tab-line-mode
-@cindex tab line
-  The command @code{global-tab-line-mode} toggles the display of a
-@dfn{tab line} on the top screen line of each window.  The tab line
-shows special buttons (``tabs'') for each buffer that was displayed in
-a window, and allows switching to any of these buffers by clicking the
-corresponding button.  You can add a tab by clicking on the @kbd{+}
-icon and delete a tab by clicking on the @kbd{x} icon of a tab.  The
-mouse wheel on the tab line scrolls the tabs horizontally.
-
 @findex winner-mode
 @vindex winner-dont-bind-my-keys
 @vindex winner-ring-size
@@ -616,3 +607,29 @@ shown in different windows.  @xref{Comparing Files}.
   Scroll All mode (@kbd{M-x scroll-all-mode}) is a global minor mode
 that causes scrolling commands and point motion commands to apply to
 every single window.
+
+
+@node Tab Line
+@section Window Tab Line
+
+@findex global-tab-line-mode
+@cindex tab line
+  The command @code{global-tab-line-mode} toggles the display of
+a @dfn{tab line} on the top screen line of each window.  The Tab Line
+shows special buttons (``tabs'') for each buffer that was displayed in
+a window, and allows switching to any of these buffers by clicking the
+corresponding button.  Clicking on the @kbd{+} icon adds a new buffer
+to the window-local tab line of buffers, and clicking on the @kbd{x}
+icon of a tab deletes it.  The mouse wheel on the tab line scrolls
+the tabs horizontally.
+
+Selecting the previous window-local tab is the same as typing @kbd{C-x
+@key{LEFT}} (@code{previous-buffer}), selecting the next tab is the
+same as @kbd{C-x @key{RIGHT}} (@code{next-buffer}).  Both commands
+support a numeric prefix argument as a repeat count.
+
+Note that the Tab Line is different from the Tab Bar (@pxref{Tab Bars}).
+Whereas tabs on the Tab Bar at the top of each frame are used to
+switch between window configurations containing several windows,
+tabs on the Tab Line at the top of each window are used to switch
+between buffers.
diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index e01cfedc4b8..e53f0e9f60c 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -4155,8 +4155,8 @@ topmost and bottommost buffer text line; and @code{top-bottom}
 indicates where there is just one line of text in the buffer.
 
 @item @code{empty-line}
-Used to indicate empty lines when @code{indicate-empty-lines} is
-non-@code{nil}.
+Used to indicate empty lines after the buffer end when
+@code{indicate-empty-lines} is non-@code{nil}.
 
 @item @code{overlay-arrow}
 Used for overlay arrows (@pxref{Overlay Arrow}).
@@ -4298,6 +4298,7 @@ The former is used by overlay arrows.  The latter is unused.
 @itemx @code{bottom-left-angle}, @code{bottom-right-angle}
 @itemx @code{top-left-angle}, @code{top-right-angle}
 @itemx @code{left-bracket}, @code{right-bracket}
+@itemx @code{empty-line}
 Used to indicate buffer boundaries.
 
 @item @code{filled-rectangle}, @code{hollow-rectangle}
@@ -4305,7 +4306,7 @@ Used to indicate buffer boundaries.
 @itemx @code{vertical-bar}, @code{horizontal-bar}
 Used for different types of fringe cursors.
 
-@item @code{empty-line}, @code{exclamation-mark}, @code{question-mark}
+@item @code{exclamation-mark}, @code{question-mark}
 Not used by core Emacs features.
 @end table
 
@@ -4338,7 +4339,8 @@ The argument @var{bits} specifies the image to use.  It should be
 either a string or a vector of integers, where each element (an
 integer) corresponds to one row of the bitmap.  Each bit of an integer
 corresponds to one pixel of the bitmap, where the low bit corresponds
-to the rightmost pixel of the bitmap.
+to the rightmost pixel of the bitmap.  (Note that this order of bits
+is opposite of the order in XBM images; @pxref{XBM Images}.)
 
 The height is normally the length of @var{bits}.  However, you
 can specify a different height with non-@code{nil} @var{height}.  The width
diff --git a/doc/lispref/eval.texi b/doc/lispref/eval.texi
index 021604c5142..baddce4d9c9 100644
--- a/doc/lispref/eval.texi
+++ b/doc/lispref/eval.texi
@@ -606,6 +606,12 @@ Here are some examples of expressions that use @code{quote}:
 @end group
 @end example
 
+  Although the expressions @code{(list '+ 1 2)} and @code{'(+ 1 2)}
+both yield lists equal to @code{(+ 1 2)}, the former yields a
+freshly-minted mutable list whereas the latter yields a constant list
+built from conses that may be shared with other constants.
+@xref{Constants and Mutability}.
+
   Other quoting constructs include @code{function} (@pxref{Anonymous
 Functions}), which causes an anonymous lambda expression written in Lisp
 to be compiled, and @samp{`} (@pxref{Backquote}), which is used to quote
diff --git a/doc/lispref/lists.texi b/doc/lispref/lists.texi
index 1125af7bec3..ea44e01f48a 100644
--- a/doc/lispref/lists.texi
+++ b/doc/lispref/lists.texi
@@ -1625,10 +1625,9 @@ keys may not be symbols:
       '(("simple leaves" . oak)
         ("compound leaves" . horsechestnut)))
 
-;; @r{The @code{copy-sequence} means the keys are not @code{eq}.}
-(assq (copy-sequence "simple leaves") leaves)
-     @result{} nil
-(assoc (copy-sequence "simple leaves") leaves)
+(assq "simple leaves" leaves)
+     @result{} @r{Unspecified; might be @code{nil} or non-@code{nil}.}
+(assoc "simple leaves" leaves)
      @result{} ("simple leaves" . oak)
 @end smallexample
 @end defun
diff --git a/doc/lispref/objects.texi b/doc/lispref/objects.texi
index 3e9f2221020..d35a9ace572 100644
--- a/doc/lispref/objects.texi
+++ b/doc/lispref/objects.texi
@@ -46,7 +46,7 @@ you store in it, type and all.  (Actually, a small number of Emacs
 Lisp variables can only take on values of a certain type.
 @xref{Variables with Restricted Values}.)
 
-  Some Lisp objects are @dfn{constant}: their values never change.
+  Some Lisp objects are @dfn{constant}: their values should never change.
 Others are @dfn{mutable}: their values can be changed via destructive
 operations that involve side effects.
 
@@ -2388,22 +2388,28 @@ that for two strings to be equal, they have the same text properties.
 @cindex constants
 @cindex mutable objects
 
-  Some Lisp objects are constant: their values never change.
+  Some Lisp objects are constant: their values should never change
+during a single execution of Emacs running well-behaved Lisp code.
 For example, you can create a new integer by calculating one, but you
 cannot modify the value of an existing integer.
 
-  Other Lisp objects are mutable: their values can be changed
+  Other Lisp objects are mutable: it is safe to change their values
 via destructive operations involving side effects.  For example, an
 existing marker can be changed by moving the marker to point to
 somewhere else.
 
-  Although numbers are always constants and markers are always
+  Although all numbers are constants and all markers are
 mutable, some types contain both constant and mutable members.  These
 types include conses, vectors, strings, and symbols.  For example, the string
 literal @code{"aaa"} yields a constant string, whereas the function
 call @code{(make-string 3 ?a)} yields a mutable string that can be
 changed via later calls to @code{aset}.
 
+  A mutable object can become constant if it is part of an expression
+that is evaluated, because a program should not modify an object
+that is being evaluated.  The reverse does not occur: constant objects
+should stay constant.
+
   Trying to modify a constant variable signals an error
 (@pxref{Constant Variables}).
 A program should not attempt to modify other types of constants because the
@@ -2411,9 +2417,10 @@ resulting behavior is undefined: the Lisp interpreter might or might
 not detect the error, and if it does not detect the error the
 interpreter can behave unpredictably thereafter.  Another way to put
 this is that although mutable objects are safe to change and constant
-symbols reliably reject attempts to change them, other constants are
-not safely mutable: if you try to change one your program might
-behave as you expect but it might crash or worse.  This problem occurs
+variables reliably prevent attempts to change them, other constants
+are not safely mutable: if a misbehaving program tries to change such a
+constant then the constant's value might actually change, or the
+program might crash or worse.  This problem occurs
 with types that have both constant and mutable members, and that have
 mutators like @code{setcar} and @code{aset} that are valid on mutable
 objects but hazardous on constants.
diff --git a/doc/lispref/tips.texi b/doc/lispref/tips.texi
index 1ca97e2f092..3b8da35b6cc 100644
--- a/doc/lispref/tips.texi
+++ b/doc/lispref/tips.texi
@@ -8,6 +8,7 @@
 @cindex tips for writing Lisp
 @cindex standards of coding style
 @cindex coding standards
+@cindex best practices
 
   This chapter describes no additional features of Emacs Lisp.  Instead
 it gives advice on making effective use of the features described in the
diff --git a/etc/NEWS.27 b/etc/NEWS.27
index 091c831e9d6..1eb391f1350 100644
--- a/etc/NEWS.27
+++ b/etc/NEWS.27
@@ -866,6 +866,11 @@ its functions.
 *** A new user option 'doc-view-pdftotext-program-args' has been added
 to allow controlling how the conversion to text is done.
 
++++
+*** The prefix key 's' was changed to 'c' for slicing commands
+to avoid conflicts with image-mode key 's'.  The new key 'c' still
+has good mnemonics of "cut", "clip", "crop".
+
 ** Ido
 
 ---
@@ -2720,8 +2725,8 @@ left to higher-level functions.
 +++
 *** Tab Bar mode
 The new command 'tab-bar-mode' enables the tab bar at the top of each
-frame, where you can use tabs to switch between named persistent
-window configurations.
+frame (including TTY frames), where you can use tabs to switch between
+named persistent window configurations.
 
 The 'C-x t' sequence is the new prefix key for tab-related commands:
 'C-x t 2' creates a new tab; 'C-x t 0' deletes the current tab;
@@ -2738,6 +2743,11 @@ when its value is "on", "yes" or "1".
 
 The user option 'tab-bar-position' specifies where to show the tab bar.
 
+Tab-related commands can be used even without the tab bar when
+'tab-bar-mode' is disabled by a nil value of the user option
+'tab-bar-show'.  Without the tab bar you can switch between tabs
+using completion on tab names, or using 'tab-switcher'.
+
 Read the new Info node "(emacs) Tab Bars" for full description
 of all related features.
 
@@ -2752,6 +2762,9 @@ a repeat count.  Clicking on the plus icon adds a new buffer to the
 window-local tab line of buffers.  Using the mouse wheel on the tab
 line scrolls tabs.
 
+Read the new Info node "(emacs) Tab Line" for full description
+of all related features.
+
 +++
 ** fileloop.el lets one setup multifile operations like search&replace.
 
@@ -3515,9 +3528,32 @@ functions.
 *** 'image-mode' now uses this library to automatically rotate images
 according to the orientation in the Exif data, if any.
 
++++
+*** The command 'image-rotate' now accepts a prefix argument.
+With a prefix argument, 'image-rotate' now rotates the image at point
+90 degrees counter-clockwise, instead of the default clockwise.
+
++++
 *** In 'image-mode' the image is resized automatically to fit in window.
-The image will resize upon first display and whenever the window's
-dimensions change.
+By default, the image will resize upon first display and whenever the
+window's dimensions change.  Two user options 'image-auto-resize' and
+'image-auto-resize-on-window-resize' control the resizing behavior
+(including the possibility to disable auto-resizing).  A new key
+prefix 's' contains the commands that can be used to fit the image to
+the window manually.
+
+---
+*** Some 'image-mode' variables are now buffer-local.
+The image parameters 'image-transform-rotation',
+'image-transform-scale' and 'image-transform-resize' are now declared
+buffer-local, so each buffer could have its own values for these
+parameters.
+
++++
+*** Three new 'image-mode' commands have been added: 'm', which marks
+the file in the dired buffer(s) for the directory the file is in; 'u',
+which unmarks the file; and 'w', which pushes the current buffer's file
+name to the kill ring.
 
 ---
 *** New library image-converter.
@@ -3538,26 +3574,6 @@ These now default to using 'image-mode'.
 some years back.  It now respects 'imagemagick-types-inhibit' as a way
 to disable that.
 
----
-*** Some 'image-mode' variables are now buffer-local.
-The image parameters 'image-transform-rotation',
-'image-transform-scale' and 'image-transform-resize' are now declared
-buffer-local, so each buffer could have its own values for these
-parameters.
-
-+++
-*** Three new 'image-mode' commands have been added: 'm', which marks
-the file in the dired buffer(s) for the directory the file is in; 'u',
-which unmarks the file; and 'w', which pushes the current buffer's file
-name to the kill ring.
-
-+++
-*** The command 'image-rotate' now accepts a prefix argument.
-With a prefix argument, 'image-rotate' now rotates the image at point
-90 degrees counter-clockwise, instead of the default clockwise.
-
-*** 'image-mode' has a new key prefix 's' for transformation commands.
-
 ** Modules
 
 ---
diff --git a/lisp/calc/calc-ext.el b/lisp/calc/calc-ext.el
index bc70ec283fa..5c11554d5d7 100644
--- a/lisp/calc/calc-ext.el
+++ b/lisp/calc/calc-ext.el
@@ -674,7 +674,6 @@
   (define-key calc-mode-map "Z/" 'calc-kbd-break)
   (define-key calc-mode-map "Z`" 'calc-kbd-push)
   (define-key calc-mode-map "Z'" 'calc-kbd-pop)
-  (define-key calc-mode-map "Z=" 'calc-kbd-report)
   (define-key calc-mode-map "Z#" 'calc-kbd-query)
 
   (calc-init-prefixes)
@@ -845,8 +844,8 @@ math-bernoulli-number math-gammap1-raw)
  ("calc-incom" calc-digit-dots)
 
  ("calc-keypd" calc-do-keypad
-calc-keypad-x-left-click calc-keypad-x-middle-click
-calc-keypad-x-right-click)
+calc-keypad-left-click calc-keypad-middle-click
+calc-keypad-right-click)
 
  ("calc-lang" calc-set-language
 math-read-big-balance math-read-big-rec)
@@ -1003,7 +1002,7 @@ calc-find-root calc-poly-interp)
 calc-floor calc-idiv calc-increment calc-mant-part calc-max calc-min
 calc-round calc-scale-float calc-sign calc-trunc calc-xpon-part)
 
- ("calc-bin" calc-and calc-binary-radix calc-clip calc-twos-complement-mode
+ ("calc-bin" calc-and calc-binary-radix calc-clip
 calc-decimal-radix calc-diff calc-hex-radix calc-leading-zeros
 calc-lshift-arith calc-lshift-binary calc-not calc-octal-radix calc-or calc-radix
 calc-rotate-binary calc-rshift-arith calc-rshift-binary calc-word-size
@@ -1116,7 +1115,7 @@ calc-equal-to calc-get-user-defn calc-greater-equal calc-greater-than
 calc-in-set calc-kbd-break calc-kbd-else calc-kbd-else-if
 calc-kbd-end-for calc-kbd-end-if calc-kbd-end-loop calc-kbd-end-repeat
 calc-kbd-for calc-kbd-if calc-kbd-loop calc-kbd-pop calc-kbd-push
-calc-kbd-query calc-kbd-repeat calc-kbd-report calc-less-equal
+calc-kbd-query calc-kbd-repeat calc-less-equal
 calc-less-than calc-logical-and calc-logical-if calc-logical-not
 calc-logical-or calc-not-equal-to calc-pass-errors calc-remove-equal
 calc-timing calc-user-define calc-user-define-composition
diff --git a/lisp/calc/calc-prog.el b/lisp/calc/calc-prog.el
index e88aa92364c..6db5de4c96c 100644
--- a/lisp/calc/calc-prog.el
+++ b/lisp/calc/calc-prog.el
@@ -1452,11 +1452,6 @@ Redefine the corresponding command."
     (error "%s" "Unbalanced Z' in keyboard macro")))
 
 
-;; (defun calc-kbd-report (msg)
-;;   (interactive "sMessage: ")
-;;   (calc-wrapper
-;;    (math-working msg (calc-top-n 1))))
-
 (defun calc-kbd-query ()
   (interactive)
   (let ((defining-kbd-macro nil)
diff --git a/lisp/doc-view.el b/lisp/doc-view.el
index 8b3d5527f08..3cac2629a9c 100644
--- a/lisp/doc-view.el
+++ b/lisp/doc-view.el
@@ -59,16 +59,16 @@
 ;; will be remembered and applied to all pages of the current
 ;; document.  This enables you to cut away the margins of a document
 ;; to save some space.  To select a slice you can use
-;; `doc-view-set-slice' (bound to `s s') which will query you for the
+;; `doc-view-set-slice' (bound to `c s') which will query you for the
 ;; coordinates of the slice's top-left corner and its width and
 ;; height.  A much more convenient way to do the same is offered by
-;; the command `doc-view-set-slice-using-mouse' (bound to `s m').
+;; the command `doc-view-set-slice-using-mouse' (bound to `c m').
 ;; After invocation you only have to press mouse-1 at the top-left
 ;; corner and drag it to the bottom-right corner of the desired slice.
 ;; Even more accurate and convenient is to use
-;; `doc-view-set-slice-from-bounding-box' (bound to `s b') which uses
+;; `doc-view-set-slice-from-bounding-box' (bound to `c b') which uses
 ;; the BoundingBox information of the current page to set an optimal
-;; slice.  To reset the slice use `doc-view-reset-slice' (bound to `s
+;; slice.  To reset the slice use `doc-view-reset-slice' (bound to `c
 ;; r').
 ;;
 ;; You can also search within the document.  The command `doc-view-search'
@@ -155,9 +155,19 @@
 (defcustom doc-view-ghostscript-program
   (cond
    ((memq system-type '(windows-nt ms-dos))
-    "gswin32c")
-   (t
-    "gs"))
+    (cond
+     ;; Windows Ghostscript
+     ((executable-find "gswin64c") "gswin64c")
+     ((executable-find "gswin32c") "gswin32c")
+     ;; The GS wrapper coming with TeX Live
+     ((executable-find "rungs") "rungs")
+     ;; The MikTeX builtin GS Check if mgs is functional for external
+     ;; non-MikTeX apps.  Was available under:
+     ;; http://blog.miktex.org/post/2005/04/07/Starting-mgsexe-at-the-DOS-Prompt.aspx
+     ((and (executable-find "mgs")
+           (= 0 (shell-command "mgs -q -dNODISPLAY -c quit")))
+      "mgs")))
+   (t "gs"))
   "Program to convert PS and PDF files to PNG."
   :type 'file
   :version "27.1")
@@ -421,10 +431,10 @@ Typically \"page-%s.png\".")
     ;; Killing the buffer (and the process)
     (define-key map (kbd "K")         'doc-view-kill-proc)
     ;; Slicing the image
-    (define-key map (kbd "s s")       'doc-view-set-slice)
-    (define-key map (kbd "s m")       'doc-view-set-slice-using-mouse)
-    (define-key map (kbd "s b")       'doc-view-set-slice-from-bounding-box)
-    (define-key map (kbd "s r")       'doc-view-reset-slice)
+    (define-key map (kbd "c s")       'doc-view-set-slice)
+    (define-key map (kbd "c m")       'doc-view-set-slice-using-mouse)
+    (define-key map (kbd "c b")       'doc-view-set-slice-from-bounding-box)
+    (define-key map (kbd "c r")       'doc-view-reset-slice)
     ;; Searching
     (define-key map (kbd "C-s")       'doc-view-search)
     (define-key map (kbd "<find>")    'doc-view-search)
diff --git a/lisp/emacs-lisp/trace.el b/lisp/emacs-lisp/trace.el
index 5c35036a50d..4ebb7ff711d 100644
--- a/lisp/emacs-lisp/trace.el
+++ b/lisp/emacs-lisp/trace.el
@@ -292,7 +292,9 @@ If `current-prefix-arg' is non-nil, also read a buffer and a \"context\"
 (defun trace-function-foreground (function &optional buffer context)
   "Trace calls to function FUNCTION.
 With a prefix argument, also prompt for the trace buffer (default
-`trace-buffer'), and a Lisp expression CONTEXT.
+`trace-buffer'), and a Lisp expression CONTEXT.  When called from
+Lisp, CONTEXT should be a function of no arguments which returns
+a value to insert into BUFFER during the trace.
 
 Tracing a function causes every call to that function to insert
 into BUFFER Lisp-style trace messages that display the function's
diff --git a/lisp/gnus/mml.el b/lisp/gnus/mml.el
index 2006837d6a7..21491499eb8 100644
--- a/lisp/gnus/mml.el
+++ b/lisp/gnus/mml.el
@@ -281,7 +281,7 @@ part.  This is for the internal use, you should never modify the value.")
             (setq tag (mml-read-tag)
                   no-markup-p nil
                   warn nil)
-          (setq tag (list 'part '(type . "text/plain"))
+          (setq tag (list 'part (cons 'type "text/plain"))
                 no-markup-p t
                 warn t))
         (setq raw (cdr (assq 'raw tag))
diff --git a/lisp/image-mode.el b/lisp/image-mode.el
index fbce1193cd0..3ee185a0dc5 100644
--- a/lisp/image-mode.el
+++ b/lisp/image-mode.el
@@ -53,11 +53,37 @@ See `image-mode-winprops'.")
   "Special hook run when image data is requested in a new window.
 It is called with one argument, the initial WINPROPS.")
 
-;; FIXME this doesn't seem mature yet. Document in manual when it is.
+(defcustom image-auto-resize t
+  "Non-nil to resize the image upon first display.
+Its value should be one of the following:
+ - nil, meaning no resizing.
+ - t, meaning to fit the image to the window height and width.
+ - `fit-height', meaning to fit the image to the window height.
+ - `fit-width', meaning to fit the image to the window width.
+ - A number, which is a scale factor (the default size is 1)."
+  :type '(choice (const :tag "No resizing" nil)
+                 (other :tag "Fit height and width" t)
+                 (const :tag "Fit height" fit-height)
+                 (const :tag "Fit width" fit-width)
+                 (number :tag "Scale factor" 1))
+  :version "27.1"
+  :group 'image)
+
+(defcustom image-auto-resize-on-window-resize 1
+  "Non-nil to resize the image whenever the window's dimensions change.
+This will always keep the image fit to the window.
+When non-nil, the value should be a number of seconds to wait before
+resizing according to the value specified in `image-auto-resize'."
+  :type '(choice (const :tag "No auto-resize on window size change" nil)
+                 (integer :tag "Wait for number of seconds before resize" 1))
+  :version "27.1"
+  :group 'image)
+
 (defvar-local image-transform-resize nil
   "The image resize operation.
 Its value should be one of the following:
  - nil, meaning no resizing.
+ - t, meaning to fit the image to the window height and width.
  - `fit-height', meaning to fit the image to the window height.
  - `fit-width', meaning to fit the image to the window width.
  - A number, which is a scale factor (the default size is 1).")
@@ -427,9 +453,10 @@ call."
     (define-key map "sf" 'image-mode-fit-frame)
     (define-key map "sh" 'image-transform-fit-to-height)
     (define-key map "sw" 'image-transform-fit-to-width)
+    (define-key map "sb" 'image-transform-fit-both)
+    (define-key map "ss" 'image-transform-set-scale)
     (define-key map "sr" 'image-transform-set-rotation)
     (define-key map "s0" 'image-transform-reset)
-    (define-key map "ss" 'image-transform-set-scale)
 
     ;; Multi-frame keys
     (define-key map (kbd "RET") 'image-toggle-animation)
@@ -482,6 +509,10 @@ call."
          :help "Resize image to match the window height"]
         ["Fit to Window Width" image-transform-fit-to-width
          :help "Resize image to match the window width"]
+        ["Fit to Window Height and Width" image-transform-fit-both
+         :help "Resize image to match the window height and width"]
+        ["Set Scale..." image-transform-set-scale
+         :help "Resize image by specified scale factor"]
         ["Rotate Image..." image-transform-set-rotation
          :help "Rotate the image"]
         ["Reset Transformations" image-transform-reset
@@ -569,6 +600,7 @@ Key bindings:
 
   (major-mode-suspend)
   (setq major-mode 'image-mode)
+  (setq image-transform-resize image-auto-resize)
 
   (if (not (image-get-display-property))
       (progn
@@ -611,7 +643,8 @@ Key bindings:
 
   (add-hook 'change-major-mode-hook #'image-toggle-display-text nil t)
   (add-hook 'after-revert-hook #'image-after-revert-hook nil t)
-  (add-hook 'window-state-change-functions #'image--window-state-change nil t)
+  (when image-auto-resize-on-window-resize
+    (add-hook 'window-state-change-functions #'image--window-state-change nil t))
 
   (run-mode-hooks 'image-mode-hook)
   (let ((image (image-get-display-property))
@@ -768,7 +801,7 @@ was inserted."
             filename))
          ;; If we have a `fit-width' or a `fit-height', don't limit
          ;; the size of the image to the window size.
-         (edges (and (null image-transform-resize)
+         (edges (and (eq image-transform-resize t)
                      (window-inside-pixel-edges (get-buffer-window))))
          (type (if (image--imagemagick-wanted-p filename)
                    'imagemagick
@@ -878,7 +911,9 @@ Otherwise, display the image by calling `image-mode'."
   ;; image resizing happens later during redisplay.  So if those
   ;; consecutive calls happen without any redisplay between them,
   ;; the costly operation of image resizing should happen only once.
-  (run-with-idle-timer 1 nil #'image-fit-to-window window))
+  (when (numberp image-auto-resize-on-window-resize)
+    (run-with-idle-timer image-auto-resize-on-window-resize nil
+                         #'image-fit-to-window window)))
 
 (defun image-fit-to-window (window)
   "Adjust size of image to display it exactly in WINDOW boundaries."
@@ -1282,7 +1317,7 @@ These properties are determined by the Image mode variables
 `image-transform-resize' and `image-transform-rotation'.  The
 return value is suitable for appending to an image spec."
   (setq image-transform-scale 1.0)
-  (when (or image-transform-resize
+  (when (or (not (memq image-transform-resize '(nil t)))
             (/= image-transform-rotation 0.0))
     ;; Note: `image-size' looks up and thus caches the untransformed
     ;; image.  There's no easy way to prevent that.
@@ -1328,6 +1363,12 @@ return value is suitable for appending to an image spec."
   (setq image-transform-resize 'fit-width)
   (image-toggle-display-image))
 
+(defun image-transform-fit-both ()
+  "Fit the current image both to the height and width of the current window."
+  (interactive)
+  (setq image-transform-resize t)
+  (image-toggle-display-image))
+
 (defun image-transform-set-rotation (rotation)
   "Prompt for an angle ROTATION, and rotate the image by that amount.
 ROTATION should be in degrees."
@@ -1338,7 +1379,7 @@ ROTATION should be in degrees."
 (defun image-transform-reset ()
   "Display the current image with the default size and rotation."
   (interactive)
-  (setq image-transform-resize nil
+  (setq image-transform-resize image-auto-resize
         image-transform-rotation 0.0
         image-transform-scale 1)
   (image-toggle-display-image))
diff --git a/lisp/simple.el b/lisp/simple.el
index 8bc84a9dfa0..b5ba05426f5 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -4846,7 +4846,7 @@ In Transient Mark mode, deactivate the mark.
 If `interprogram-cut-function' is non-nil, also save the text for a window
 system cut and paste.
 
-If you want to append the killed line to the last killed text,
+If you want to append the killed region to the last killed text,
 use \\[append-next-kill] before \\[kill-ring-save].
 
 The copied text is filtered by `filter-buffer-substring' before it is
diff --git a/src/xdisp.c b/src/xdisp.c
index 32588939564..140d134572f 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -30,8 +30,9 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    Updating the display is triggered by the Lisp interpreter when it
    decides it's time to do it.  This is done either automatically for
    you as part of the interpreter's command loop or as the result of
-   calling Lisp functions like `sit-for'.  The C function `redisplay'
-   in xdisp.c is the only entry into the inner redisplay code.
+   calling Lisp functions like `sit-for'.  The C function
+   `redisplay_internal' in xdisp.c is the only entry into the inner
+   redisplay code.
 
    The following diagram shows how redisplay code is invoked.  As you
    can see, Lisp calls redisplay and vice versa.
@@ -89,7 +90,15 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    second glyph matrix is constructed, the so called `desired glyph
    matrix' or short `desired matrix'.  Current and desired matrix are
    then compared to find a cheap way to update the display, e.g. by
-   reusing part of the display by scrolling lines.
+   reusing part of the display by scrolling lines.  The actual update
+   of the display of each window by comparing the desired and the
+   current matrix is done by `update_window', which calls functions
+   which draw to the glass (those functions are specific to the type
+   of the window's frame: X, w32, NS, etc.).
+
+   Once the display of a window on the glass has been updated, its
+   desired matrix is used to update the corresponding rows of the
+   current matrix, and then the desired matrix is discarded.
 
    You will find a lot of redisplay optimizations when you start
    looking at the innards of redisplay.  The overall goal of all these
@@ -119,13 +128,13 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
     . try_window
 
-      This function performs the full redisplay of a single window
-      assuming that its fonts were not changed and that the cursor
-      will not end up in the scroll margins.  (Loading fonts requires
-      re-adjustment of dimensions of glyph matrices, which makes this
-      method impossible to use.)
+      This function performs the full, unoptimized, redisplay of a
+      single window assuming that its fonts were not changed and that
+      the cursor will not end up in the scroll margins.  (Loading
+      fonts requires re-adjustment of dimensions of glyph matrices,
+      which makes this method impossible to use.)
 
-   These optimizations are tried in sequence (some can be skipped if
+   The optimizations are tried in sequence (some can be skipped if
    it is known that they are not applicable).  If none of the
    optimizations were successful, redisplay calls redisplay_windows,
    which performs a full redisplay of all windows.
@@ -145,38 +154,62 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
    Desired matrices.
 
-   Desired matrices are always built per Emacs window.  The function
-   `display_line' is the central function to look at if you are
-   interested.  It constructs one row in a desired matrix given an
+   Desired matrices are always built per Emacs window.  It is
+   important to know that a desired matrix is in general "sparse": it
+   only has some of the glyph rows "enabled".  This is because
+   redisplay tries to optimize its work, and thus only generates
+   glyphs for rows that need to be updated on the screen.  Rows that
+   don't need to be updated are left "disabled", and their contents
+   should be ignored.
+
+   The function `display_line' is the central function to look at if
+   you are interested in how the rows of the desired matrix are
+   produced.  It constructs one row in a desired matrix given an
    iterator structure containing both a buffer position and a
    description of the environment in which the text is to be
    displayed.  But this is too early, read on.
 
+   Glyph rows.
+
+   A glyph row is an array of `struct glyph', where each glyph element
+   describes a "display element" to be shown on the screen.  More
+   accurately, a glyph row can have up to 3 different arrays of
+   glyphs: one each for every display margins, and one for the "text
+   area", where buffer text is displayed.  The text-area glyph array
+   is always present, whereas the arrays for the marginal areas are
+   present (non-empty) only if the corresponding display margin is
+   shown in the window.  If the glyph array for a marginal area is not
+   present its beginning and end coincide, i.e. such arrays are
+   actually empty (they contain no glyphs).  Frame glyph matrics, used
+   on text-mode terminals (see below) never have marginal areas, they
+   treat the entire frame-wide row of glyphs as a single large "text
+   area".
+
    Iteration over buffer and strings.
 
    Characters and pixmaps displayed for a range of buffer text depend
    on various settings of buffers and windows, on overlays and text
    properties, on display tables, on selective display.  The good news
    is that all this hairy stuff is hidden behind a small set of
-   interface functions taking an iterator structure (struct it)
+   interface functions taking an iterator structure (`struct it')
    argument.
 
    Iteration over things to be displayed is then simple.  It is
-   started by initializing an iterator with a call to init_iterator,
+   started by initializing an iterator with a call to `init_iterator',
    passing it the buffer position where to start iteration.  For
-   iteration over strings, pass -1 as the position to init_iterator,
-   and call reseat_to_string when the string is ready, to initialize
+   iteration over strings, pass -1 as the position to `init_iterator',
+   and call `reseat_to_string' when the string is ready, to initialize
    the iterator for that string.  Thereafter, calls to
-   get_next_display_element fill the iterator structure with relevant
-   information about the next thing to display.  Calls to
-   set_iterator_to_next move the iterator to the next thing.
+   `get_next_display_element' fill the iterator structure with
+   relevant information about the next thing to display.  Calls to
+   `set_iterator_to_next' move the iterator to the next thing.
 
    Besides this, an iterator also contains information about the
    display environment in which glyphs for display elements are to be
    produced.  It has fields for the width and height of the display,
    the information whether long lines are truncated or continued, a
-   current X and Y position, and lots of other stuff you can better
-   see in dispextern.h.
+   current X and Y position, the face currently in effect, and lots of
+   other stuff you can better see in dispextern.h.
 
    The "stop position".
 
@@ -184,57 +217,62 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    infrequently.  These include the face of the characters, whether
    text is invisible, the object (buffer or display or overlay string)
    being iterated, character composition info, etc.  For any given
-   buffer or string position, the sources of information that
-   affects the display can be determined by calling the appropriate
-   primitives, such as Fnext_single_property_change, but both these
+   buffer or string position, the sources of information that affects
+   the display can be determined by calling the appropriate
+   primitives, such as `Fnext_single_property_change', but both these
    calls and the processing of their return values is relatively
    expensive.  To optimize redisplay, the display engine checks these
-   sources of display information only when needed.  To that end, it
-   always maintains the position of the next place where it must stop
-   and re-examine all those potential sources.  This is called "stop
-   position" and is stored in the stop_charpos field of the iterator.
-   The stop position is updated by compute_stop_pos, which is called
-   whenever the iteration reaches the current stop position and
-   processes it.  Processing a stop position is done by handle_stop,
-   which invokes a series of handlers, one each for every potential
-   source of display-related information; see the it_props array for
-   those handlers.  For example, one handler is handle_face_prop,
-   which detects changes in face properties, and supplies the face ID
-   that the iterator will use for all the glyphs it generates up to
-   the next stop position; this face ID is the result of realizing the
-   face specified by the relevant text properties at this position.
-   Each handler called by handle_stop processes the sources of display
+   sources of display information only when needed, not for every
+   character.  To that end, it always maintains the position of the
+   next place where it must stop and re-examine all those potential
+   sources.  This is called "the stop position" and is stored in the
+   `stop_charpos' field of the iterator.  The stop position is updated
+   by `compute_stop_pos', which is called whenever the iteration
+   reaches the current stop position and processes it.  Processing a
+   stop position is done by `handle_stop', which invokes a series of
+   handlers, one each for every potential source of display-related
+   information; see the `it_props' array for those handlers.  For
+   example, one handler is `handle_face_prop', which detects changes
+   in face properties, and supplies the face ID that the iterator will
+   use for all the glyphs it generates up to the next stop position;
+   this face ID is the result of "realizing" the face specified by the
+   relevant text properties at this position (see xfaces.c).  Each
+   handler called by `handle_stop' processes the sources of display
    information for which it is "responsible", and returns a value
-   which tells handle_stop what to do next.
+   which tells `handle_stop' what to do next.
+
+   Once `handle_stop' returns, the information it stores in the
+   iterator fields will not be refreshed until the iteration reaches
+   the next stop position, which is computed by `compute_stop_pos'
+   called at the end of `handle_stop'.  `compute_stop_pos' examines
+   the buffer's or string's interval tree to determine where the text
+   properties change, finds the next position where overlays and
+   character composition can change, and stores in `stop_charpos' the
+   closest position where any of these factors should be reconsidered.
 
-   Once handle_stop returns, the information it stores in the iterator
-   fields will not be refreshed until the iteration reaches the next
-   stop position, which is computed by compute_stop_pos called at the
-   end of handle_stop.  compute_stop_pos examines the buffer's or
-   string's interval tree to determine where the text properties
-   change, finds the next position where overlays and character
-   composition can change, and stores in stop_charpos the closest
-   position where any of these factors should be reconsidered.
+   Handling of the stop position is done as part of the code in
+   `get_next_display_element'.
 
    Producing glyphs.
 
    Glyphs in a desired matrix are normally constructed in a loop
-   calling get_next_display_element and then PRODUCE_GLYPHS.  The call
-   to PRODUCE_GLYPHS will fill the iterator structure with pixel
-   information about the element being displayed and at the same time
-   produce glyphs for it.  If the display element fits on the line
-   being displayed, set_iterator_to_next is called next, otherwise the
-   glyphs produced are discarded.  The function display_line is the
-   workhorse of filling glyph rows in the desired matrix with glyphs.
-   In addition to producing glyphs, it also handles line truncation
-   and continuation, word wrap, and cursor positioning (for the
-   latter, see also set_cursor_from_row).
+   calling `get_next_display_element' and then `PRODUCE_GLYPHS'.  The
+   call to `PRODUCE_GLYPHS' will fill the iterator structure with
+   pixel information about the element being displayed and at the same
+   time will produce glyphs for it.  If the display element fits on
+   the line being displayed, `set_iterator_to_next' is called next,
+   otherwise the glyphs produced are discarded, and `display_line'
+   marks this glyph row as a "continued line".  The function
+   `display_line' is the workhorse of filling glyph rows in the
+   desired matrix with glyphs.  In addition to producing glyphs, it
+   also handles line truncation and continuation, word wrap, and
+   cursor positioning (for the latter, see `set_cursor_from_row').
 
    Frame matrices.
 
    That just couldn't be all, could it?  What about terminal types not
    supporting operations on sub-windows of the screen (a.k.a. "TTY" or
-   "text-mode terminal")?  To update the display on such a terminal,
+   "text-mode terminals")?  To update the display on such a terminal,
    window-based glyph matrices are not well suited.  To be able to
    reuse part of the display (scrolling lines up and down), we must
    instead have a view of the whole screen.  This is what `frame
@@ -252,19 +290,62 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    using the frame matrices, which allows frame-global optimization of
    what is actually written to the glass.
 
-   To be honest, there is a little bit more done, but not much more.
-   If you plan to extend that code, take a look at dispnew.c.  The
-   function build_frame_matrix is a good starting point.
+   Frame matrices don't have marginal areas, only a text area.  That
+   is, the entire row of glyphs that spans the width of a text-mode
+   frame is treated as a single large "text area" for the purposes of
+   manipulating and updating a frame glyph matrix.
+
+   To be honest, there is a little bit more done for frame matrices,
+   but not much more.  If you plan to extend that code, take a look at
+   dispnew.c.  The function build_frame_matrix is a good starting
+   point.
+
+   Simulating display.
+
+   Some of Emacs commands and functions need to take display layout
+   into consideration.  For example, C-n moves to the next screen
+   line, but to implement that, Emacs needs to find the buffer
+   position which is directly below the cursor position on display.
+   This is not trivial when buffer display includes variable-size
+   elements such as different fonts, tall images, etc.
+
+   To solve this problem, the display engine implements several
+   functions that can move through buffer text in the same manner as
+   `display_line' and `display_string' do, but without producing any
+   glyphs for the glyph matrices.  The workhorse of this is
+   `move_it_in_display_line_to'.  Its code and logic are very similar
+   to `display_line', but it differs in two important aspects: it
+   doesn't produce glyphs for any glyph matrix, and it returns a
+   status telling the caller how it ended the iteration: whether it
+   reached the required position, hit the end of line, arrived at the
+   window edge without exhausting the buffer's line, etc.  Since the
+   glyphs are not produced, the layout information available to the
+   callers of this function is what is recorded in `struct it' by the
+   iteration process.
+
+   Several higher-level functions call `move_it_in_display_line_to' to
+   perform more complex tasks: `move_it_by_lines' can move N lines up
+   or down from a given buffer position and `move_it_to' can move to a
+   given buffer position or to a given X or Y pixel coordinate.
+
+   These functions are called by the display engine itself as well,
+   when it needs to make layout decisions before producing the glyphs.
+   For example, one of the first things to decide when redisplaying a
+   window is where to put the `window-start' position; if the window
+   is to be recentered (the default), Emacs makes that decision by
+   starting from the position of point, then moving up the number of
+   lines corresponding to half the window height using
+   `move_it_by_lines'.
 
    Bidirectional display.
 
    Bidirectional display adds quite some hair to this already complex
    design.  The good news are that a large portion of that hairy stuff
    is hidden in bidi.c behind only 3 interfaces.  bidi.c implements a
-   reordering engine which is called by set_iterator_to_next and
+   reordering engine which is called by `set_iterator_to_next' and
    returns the next character to display in the visual order.  See
    commentary on bidi.c for more details.  As far as redisplay is
-   concerned, the effect of calling bidi_move_to_visually_next, the
+   concerned, the effect of calling `bidi_move_to_visually_next', the
    main interface of the reordering engine, is that the iterator gets
    magically placed on the buffer or string position that is to be
    displayed next in the visual order.  In other words, a linear
@@ -279,27 +360,27 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    monotonously changing with vertical positions.  Also, accounting
    for face changes, overlays, etc. becomes more complex because
    non-linear iteration could potentially skip many positions with
-   changes, and then cross them again on the way back (see
-   handle_stop_backwards)...
+   such changes, and then cross them again on the way back (see
+   `handle_stop_backwards')...
 
    One other prominent effect of bidirectional display is that some
    paragraphs of text need to be displayed starting at the right
    margin of the window---the so-called right-to-left, or R2L
    paragraphs.  R2L paragraphs are displayed with R2L glyph rows,
-   which have their reversed_p flag set.  The bidi reordering engine
+   which have their `reversed_p' flag set.  The bidi reordering engine
    produces characters in such rows starting from the character which
-   should be the rightmost on display.  PRODUCE_GLYPHS then reverses
-   the order, when it fills up the glyph row whose reversed_p flag is
-   set, by prepending each new glyph to what is already there, instead
-   of appending it.  When the glyph row is complete, the function
-   extend_face_to_end_of_line fills the empty space to the left of the
-   leftmost character with special glyphs, which will display as,
-   well, empty.  On text terminals, these special glyphs are simply
-   blank characters.  On graphics terminals, there's a single stretch
-   glyph of a suitably computed width.  Both the blanks and the
-   stretch glyph are given the face of the background of the line.
-   This way, the terminal-specific back-end can still draw the glyphs
-   left to right, even for R2L lines.
+   should be the rightmost on display.  `PRODUCE_GLYPHS' then reverses
+   the order, when it fills up the glyph row whose `reversed_p' flag
+   is set, by prepending each new glyph to what is already there,
+   instead of appending it.  When the glyph row is complete, the
+   function `extend_face_to_end_of_line' fills the empty space to the
+   left of the leftmost character with special glyphs, which will
+   display as, well, empty.  On text terminals, these special glyphs
+   are simply blank characters.  On graphics terminals, there's a
+   single stretch glyph of a suitably computed width.  Both the blanks
+   and the stretch glyph are given the face of the background of the
+   line.  This way, the terminal-specific back-end can still draw the
+   glyphs left to right, even for R2L lines.
 
    Bidirectional display and character compositions.
 
@@ -310,23 +391,23 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 
    Emacs display supports this by providing "character compositions",
    most of which is implemented in composite.c.  During the buffer
-   scan that delivers characters to PRODUCE_GLYPHS, if the next
+   scan that delivers characters to `PRODUCE_GLYPHS', if the next
    character to be delivered is a composed character, the iteration
-   calls composition_reseat_it and next_element_from_composition.  If
-   they succeed to compose the character with one or more of the
+   calls `composition_reseat_it' and `next_element_from_composition'.
+   If they succeed to compose the character with one or more of the
    following characters, the whole sequence of characters that were
    composed is recorded in the `struct composition_it' object that is
    part of the buffer iterator.  The composed sequence could produce
    one or more font glyphs (called "grapheme clusters") on the screen.
-   Each of these grapheme clusters is then delivered to PRODUCE_GLYPHS
-   in the direction corresponding to the current bidi scan direction
-   (recorded in the scan_dir member of the `struct bidi_it' object
-   that is part of the iterator).  In particular, if the bidi iterator
-   currently scans the buffer backwards, the grapheme clusters are
-   delivered back to front.  This reorders the grapheme clusters as
-   appropriate for the current bidi context.  Note that this means
-   that the grapheme clusters are always stored in the LGSTRING object
-   (see composite.c) in the logical order.
+   Each of these grapheme clusters is then delivered to
+   `PRODUCE_GLYPHS' in the direction corresponding to the current bidi
+   scan direction (recorded in the `scan_dir' member of the `struct
+   bidi_it' object that is part of the iterator).  In particular, if
+   the bidi iterator currently scans the buffer backwards, the
+   grapheme clusters are delivered back to front.  This reorders the
+   grapheme clusters as appropriate for the current bidi context.
+   Note that this means that the grapheme clusters are always stored
+   in the `LGSTRING' object (see composite.c) in the logical order.
 
    Moving an iterator in bidirectional text
    without producing glyphs.
@@ -337,18 +418,18 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
    As far as the iterator is concerned, the geometry of such rows is
    still left to right, i.e. the iterator "thinks" the first character
    is at the leftmost pixel position.  The iterator does not know that
-   PRODUCE_GLYPHS reverses the order of the glyphs that the iterator
-   delivers.  This is important when functions from the move_it_*
+   `PRODUCE_GLYPHS' reverses the order of the glyphs that the iterator
+   delivers.  This is important when functions from the `move_it_*'
    family are used to get to certain screen position or to match
    screen coordinates with buffer coordinates: these functions use the
    iterator geometry, which is left to right even in R2L paragraphs.
-   This works well with most callers of move_it_*, because they need
+   This works well with most callers of `move_it_*', because they need
    to get to a specific column, and columns are still numbered in the
    reading order, i.e. the rightmost character in a R2L paragraph is
    still column zero.  But some callers do not get well with this; a
    notable example is mouse clicks that need to find the character
    that corresponds to certain pixel coordinates.  See
-   buffer_posn_from_coords in dispnew.c for how this is handled.  */
+   `buffer_posn_from_coords' in dispnew.c for how this is handled.  */
 
 #include <config.h>
 #include <stdlib.h>