Display separators on tty menus with display table entries

* src/xdisp.c (display_tty_menu_separator): Lookup separator char in
standard-display-table, make a string, and display that using
display_string.
* src/xdisp.c (display_tty_menu_separator_char): New function.
(display_tty_menu_separator): Use it.
* lisp/disp-table.el (display-table): Increase from 12 to 18.
(box-double-vertical, box-double-horizontal, box-double-down-right)
(box-double-down-left, box-double-up-right, box-double-up-left): New
symbols for extra slots.
(display-table-slot, set-display-table-slot): Change doc string.
(describe-display-table): Describe new slots.
(standard-display-unicode-special-glyphs): Define new slots.
* src/disptab.h (DISP_TABLE_P): Add enumerators.
(DISP_TABLE_EXTRA_SLOTS): Define based on enum box.
* src/dispnew.c (produce_box_glyphs): Add new enumerators to switch to
make it exhaustive.
* src/xdisp.c (display_tty_menu_separator): Use BOX_DOUBLE_HORIZONTAL
for '=' if present.
* doc/lispref/display.texi: Add documentation.
* etc/NEWS: Mention in NEWS.

1 files changed, 30 insertions, 6 deletions

diff --git a/doc/lispref/display.texi b/doc/lispref/display.texi
index b17ee2e96a1..39f182995c0 100644
--- a/doc/lispref/display.texi
+++ b/doc/lispref/display.texi
@@ -8715,6 +8715,20 @@ scroll bars are supported and in use, a scroll bar separates the two
 windows, and if there are no vertical scroll bars and no dividers
 (@pxref{Window Dividers}), Emacs uses a thin line to indicate the
 border.
+
+@item 6 to 11
+
+The glyphs for a single-line border around child frames on a terminal,
+in the order of vertical, horizontal, down-right edge, down-left edge,
+up-right, and up-left edge glyphs.  The horizontal glyph is also used for
+the single-line tty menu separator.
+
+@item 12 to 17
+
+The glyphs for a double-line border, in the order of vertical,
+horizontal, down-right edge, down-left edge, up-right, and up-left edge
+glyphs. The horizontal glyph is also used for the single-line tty menu
+separator, the other glyphs are not yet used.
 @end table
 
   For example, here is how to construct a display table that mimics
@@ -8737,24 +8751,34 @@ the effect of setting @code{ctl-arrow} to a non-@code{nil} value
 @defun display-table-slot display-table slot
 This function returns the value of the extra slot @var{slot} of
 @var{display-table}.  The argument @var{slot} may be a number from 0 to
-5 inclusive, or a slot name (symbol).  Valid symbols are
-@code{truncation}, @code{wrap}, @code{escape}, @code{control},
-@code{selective-display}, and @code{vertical-border}.
+17 inclusive, or a slot name, a symbol.
 @end defun
 
 @defun set-display-table-slot display-table slot value
 This function stores @var{value} in the extra slot @var{slot} of
 @var{display-table}.  The argument @var{slot} may be a number from 0 to
-5 inclusive, or a slot name (symbol).  Valid symbols are
-@code{truncation}, @code{wrap}, @code{escape}, @code{control},
-@code{selective-display}, and @code{vertical-border}.
+17 inclusive, or a slot name, a symbol.
 @end defun
 
+Valid slot name symbols are @code{truncation}, @code{wrap},
+@code{escape}, @code{control}, @code{selective-display},
+@code{vertical-border}, @code{box-vertical}, @code{box-horizontal},
+@code{box-down-right}, @code{box-down-left}, @code{box-up-right},
+@code{box-up-left}, @code{box-double-vertical},
+@code{box-double-horizontal}, @code{box-double-down-right},
+@code{box-double-down-left}, @code{box-double-down-left},
+@code{box-double-up-left}.
+
 @defun describe-display-table display-table
 This function displays a description of the display table
 @var{display-table} in a help buffer.
 @end defun
 
+@defun standard-display-unicode-special-glyphs
+This function sets the extra slots of @var{standard-display-table} with
+suitable Unicode characters.
+@end defun
+
 @deffn Command describe-current-display-table
 This command displays a description of the current display table in a
 help buffer.