bug#29325: 26.0.90: Info scrolling stuck

[Eli Zaretskii]

> Date: Tue, 21 Nov 2017 21:42:59 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 29325@debbugs.gnu.org
> 
> > When causing the problematic behavior, redisplay_window reaches
> > try_to_scroll via this if statement:
> > 
> >        if (!cursor_row_fully_visible_p (w, false, false))
> >     {
> >       /* Point does appear, but on a line partly visible at end of window.
> >          Move it back to a fully-visible line.  */
> >       new_vpos = window_box_height (w);
> >       /* But if window_box_height suggests a Y coordinate that is
> >          not less than we already have, that line will clearly not
> >          be fully visible, so give up and scroll the display.
> >          This can happen when the default face uses a font whose
> >          dimensions are different from the frame's default
> >          font.  */
> >       if (new_vpos >= w->cursor.y)
> >         {
> >           w->cursor.vpos = -1;
> >           clear_glyph_matrix (w->desired_matrix);
> >           goto try_to_scroll;
> >         }
> >     }
> > 
> > That is, (!cursor_row_fully_visible_p (w, false, false)) and
> > (new_vpos >= w->cursor.y) evaluate to true.
> 
> And what are the values of new_vpos and w->cursor.y in this case?  And
> what is w->cursor.vpos?
> 
> > try_to_scroll looks like this:
> 
> I suspect we shouldn't go there in this case.

Actually, I think this code is not supposed to run in the case you
describe, i.e. when the user invokes a scrolling command.  So I think
the root cause is elsewhere.  Does the patch below fix the problem?

diff --git a/src/window.c b/src/window.c
index 7f47252..504dcd3 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5355,6 +5355,9 @@ window_scroll_pixel_based (Lisp_Object window, int n, 
bool whole, bool noerror)
       /* We moved the window start towards BEGV, so PT may be now
         in the scroll margin at the bottom.  */
       move_it_to (&it, PT, -1,
+                 /* We subtract WINDOW_HEADER_LINE_HEIGHT because
+                    it.y is relative to the bottom of the header
+                    line, see above.  */
                  (it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w)
                    - partial_line_height (&it) - this_scroll_margin - 1),
                  -1,
@@ -5392,11 +5395,14 @@ window_scroll_pixel_based (Lisp_Object window, int n, 
bool whole, bool noerror)
 
       /* See if point is on a partially visible line at the end.  */
       if (it.what == IT_EOB)
-       partial_p = it.current_y + it.ascent + it.descent > it.last_visible_y;
+       partial_p =
+         it.current_y + it.ascent + it.descent
+         > it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w);
       else
        {
          move_it_by_lines (&it, 1);
-         partial_p = it.current_y > it.last_visible_y;
+         partial_p =
+           it.current_y > it.last_visible_y - WINDOW_HEADER_LINE_HEIGHT (w);
        }
 
       if (charpos == PT && !partial_p
@@ -5415,7 +5421,7 @@ window_scroll_pixel_based (Lisp_Object window, int n, 
bool whole, bool noerror)
            goal_y = this_scroll_margin;
          SET_TEXT_POS_FROM_MARKER (start, w->start);
          start_display (&it, w, start);
-         /* It would be wrong to subtract CURRENT_HEADER_LINE_HEIGHT
+         /* It would be wrong to subtract WINDOW_HEADER_LINE_HEIGHT
             here because we called start_display again and did not
             alter it.current_y this time.  */
          move_it_to (&it, -1, window_scroll_pixel_based_preserve_x,