Improve scrolling when line-spacing != 0 and scroll-step = 1.

 lisp/simple.el (default-line-height): New function.
 (line-move-partial, line-move): Use it instead of computing the
 line height inline.
 (line-move-partial): Always compute ROWH.  If the last line is
 partially-visible, but its text is completely visible, allow
 cursor to enter such a partially-visible line.

2 files changed, 55 insertions, 37 deletions

diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 008932be738..ce10b033652 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,12 @@
+2013-07-10  Eli Zaretskii  <eliz@gnu.org>
+
+        * simple.el (default-line-height): New function.
+        (line-move-partial, line-move): Use it instead of computing the
+        line height inline.
+        (line-move-partial): Always compute ROWH.  If the last line is
+        partially-visible, but its text is completely visible, allow
+        cursor to enter such a partially-visible line.
+
 2013-07-10  Michael Albinus  <michael.albinus@gmx.de>
 
         Improve error messages.  (Bug#14808)
diff --git a/lisp/simple.el b/lisp/simple.el
index 1baf0eb6d4e..9774bc0e292 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -4732,18 +4732,35 @@ lines."
     (aref (font-info (face-font 'default)) 3))
    (t (frame-char-height))))
 
+(defun default-line-height ()
+  "Return the pixel height of current buffer's default-face text line.
+
+The value includes `line-spacing', if any, defined for the buffer
+or the frame."
+  (let ((dfh (default-font-height))
+        (lsp (if (display-graphic-p)
+                 (or line-spacing
+                     (default-value 'line-spacing)
+                     (frame-parameter nil 'line-spacing)
+                     0)
+               0)))
+    (if (floatp lsp)
+        (setq lsp (* dfh lsp)))
+    (+ dfh lsp)))
+
 (defun window-screen-lines ()
   "Return the number of screen lines in the text area of the selected window.
 
 This is different from `window-text-height' in that this function counts
 lines in units of the height of the font used by the default face displayed
-in the window, not in units of the frame's default font.
+in the window, not in units of the frame's default font, and also accounts
+for `line-spacing', if any, defined for the window's buffer or frame.
 
 The value is a floating-point number."
   (let ((canonical (window-text-height))
         (fch (frame-char-height))
-        (dfh (default-font-height)))
-    (/ (* (float canonical) fch) dfh)))
+        (dlh (default-line-height)))
+    (/ (* (float canonical) fch) dlh)))
 
 ;; Returns non-nil if partial move was done.
 (defun line-move-partial (arg noerror to-end)
@@ -4751,31 +4768,24 @@ The value is a floating-point number."
       ;; Move backward (up).
       ;; If already vscrolled, reduce vscroll
       (let ((vs (window-vscroll nil t))
-            (dfh (default-font-height)))
-        (when (> vs dfh)
-          (set-window-vscroll nil (- vs dfh) t)))
+            (dlh (default-line-height)))
+        (when (> vs dlh)
+          (set-window-vscroll nil (- vs dlh) t)))
 
     ;; Move forward (down).
     (let* ((lh (window-line-height -1))
+           (rowh (car lh))
            (vpos (nth 1 lh))
            (ypos (nth 2 lh))
            (rbot (nth 3 lh))
            (this-lh (window-line-height))
-           (this-height (nth 0 this-lh))
+           (this-height (car this-lh))
            (this-ypos (nth 2 this-lh))
-           (dfh (default-font-height))
-           (lsp (if (display-graphic-p)
-                    (or line-spacing
-                        (default-value 'line-spacing)
-                        (frame-parameter nil 'line-spacing)
-                        0)
-                  0))
-           py vs rowh dlh)
-      (if (floatp lsp)
-          (setq lsp (* dfh lsp)))
-      ;; Default height of a text line, accounting for the default
-      ;; face's font and line-spacing, if any.
-      (setq dlh (+ dfh lsp))
+           (dlh (default-line-height))
+           (wslines (window-screen-lines))
+           py vs last-line)
+      (if (> (mod wslines 1.0) 0.0)
+          (setq wslines (round (+ wslines 0.5))))
       (when (or (null lh)
                 (>= rbot dlh)
                 (<= ypos (- dlh))
@@ -4798,6 +4808,19 @@ The value is a floating-point number."
                     (if col-row
                         (- (cdr col-row) (window-vscroll))
                       (cdr (posn-col-row ppos))))))
+        ;; VPOS > 0 means the last line is only partially visible.
+        ;; But if the part that is visible is at least as tall as the
+        ;; default font, that means the line is actually fully
+        ;; readable, and something like line-spacing is hidden.  So in
+        ;; that case we accept the last line in the window as still
+        ;; visible, and consider the margin as starting one line
+        ;; later.
+        (if (and vpos (> vpos 0))
+            (if (and rowh
+                     (>= rowh (default-font-height))
+                     (< rowh dlh))
+                (setq last-line (min (- wslines scroll-margin) vpos))
+              (setq last-line (min (- wslines scroll-margin 1) (1- vpos)))))
         (cond
          ;; If last line of window is fully visible, and vscrolling
          ;; more would make this line invisible, move forward.
@@ -4811,8 +4834,7 @@ The value is a floating-point number."
          ((and (or (null this-height) (<= this-height dlh))
                vpos
                (> vpos 0)
-               (< py
-                  (min (- (window-screen-lines) scroll-margin 1) (1- vpos))))
+               (< py last-line))
           nil)
          ;; When already vscrolled, we vscroll some more if we can,
          ;; or clear vscroll and move forward at end of tall image.
@@ -4824,8 +4846,7 @@ The value is a floating-point number."
          ;; but also optionally vscroll one line so redisplay won't recenter.
          ((and vpos
                (> vpos 0)
-               (= py (min (- (window-screen-lines) scroll-margin 1)
-                          (1- vpos))))
+               (= py last-line))
           ;; Don't vscroll if the partially-visible line at window
           ;; bottom has the default height (a.k.a. "just one more text
           ;; line"): in that case, we do want redisplay to behave
@@ -4880,19 +4901,7 @@ The value is a floating-point number."
             ;; If we moved into a tall line, set vscroll to make
             ;; scrolling through tall images more smooth.
             (let ((lh (line-pixel-height))
-                  (dfh (default-font-height))
-                  (lsp (if (display-graphic-p)
-                           (or line-spacing
-                               (default-value 'line-spacing)
-                               (frame-parameter nil 'line-spacing)
-                               0)
-                         0))
-                  dlh)
-              ;; DLH is the default height of a text line, accounting
-              ;; for the default face's font and line-spacing, if any.
-              (if (floatp lsp)
-                  (setq lsp (* dfh lsp)))
-              (setq dlh (+ dfh lsp))
+                  (dlh (default-line-height)))
               (if (and (< arg 0)
                        (< (point) (window-start))
                        (> lh dlh))