Fix scrolling with partial line corner case (Bug#25792)

Also fix up the scrolling tests so that they don't make so many
assumptions about the current window configuration.

* src/xdisp.c (try_window): Take partial line height into account when
comparing cursor position against scroll margin.

* test/manual/scroll-tests.el (scroll-tests-with-buffer-window): Add
HEIGHT argument, to allow setting up window with exact height and
partial line.
(scroll-tests-display-buffer-with-height): New display-buffer action
function.
(scroll-tests-scroll-margin-over-max):
(scroll-tests--scroll-margin-whole-window): Pass HEIGHT to
`scroll-tests--scroll-margin-whole-window'.
(scroll-tests-conservative-show-trailing-whitespace): New test.
(scroll-tests-scroll-margin-negative): Fix line counting.
(scroll-tests--point-in-middle-of-window-p): Set window height
properly.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index b0ff627c70e..b0644882bdc 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -17380,21 +17380,27 @@ try_window (Lisp_Object window, struct text_pos pos, int flags)
         return 0;
     }
 
+  /* Save the character position of 'it' before we call
+     'start_display' again.  */
+  ptrdiff_t it_charpos = IT_CHARPOS (it);
+
   /* Don't let the cursor end in the scroll margins.  */
   if ((flags & TRY_WINDOW_CHECK_MARGINS)
       && !MINI_WINDOW_P (w))
     {
       int this_scroll_margin = window_scroll_margin (w, MARGIN_IN_PIXELS);
+      start_display (&it, w, pos);
 
       if ((w->cursor.y >= 0     /* not vscrolled */
            && w->cursor.y < this_scroll_margin
            && CHARPOS (pos) > BEGV
-           && IT_CHARPOS (it) < ZV)
+           && it_charpos < ZV)
           /* rms: considering make_cursor_line_fully_visible_p here
              seems to give wrong results.  We don't want to recenter
              when the last line is partly visible, we want to allow
              that case to be handled in the usual way.  */
-          || w->cursor.y > it.last_visible_y - this_scroll_margin - 1)
+          || w->cursor.y > (it.last_visible_y - partial_line_height (&it)
+                            - this_scroll_margin - 1))
         {
           w->cursor.vpos = -1;
           clear_glyph_matrix (w->desired_matrix);
@@ -17403,7 +17409,7 @@ try_window (Lisp_Object window, struct text_pos pos, int flags)
     }
 
   /* If bottom moved off end of frame, change mode line percentage.  */
-  if (w->window_end_pos <= 0 && Z != IT_CHARPOS (it))
+  if (w->window_end_pos <= 0 && Z != it_charpos)
     w->update_mode_line = true;
 
   /* Set window_end_pos to the offset of the last character displayed