feature/fix-the-long-lines-display-bug 0699f80f85 1/2: Fix calculation of the vertical scroll bar's thumb

[Gregory Heytings]

branch: feature/fix-the-long-lines-display-bug
commit 0699f80f851b1f9e2f7b0a22ddd688abb7bebde1
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Gregory Heytings <gregory@heytings.org>

    Fix calculation of the vertical scroll bar's thumb
    
    * src/xdisp.c (set_vertical_scroll_bar): Compute window's end
    position "by hand" if w->window_end_pos cannot be relied upon.
---
 src/xdisp.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index 375158a520..ae6553d876 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -18924,11 +18924,27 @@ set_vertical_scroll_bar (struct window *w)
          && NILP (echo_area_buffer[0])))
     {
       struct buffer *buf = XBUFFER (w->contents);
+      ptrdiff_t window_end_pos = w->window_end_pos;
+
+      /* If w->window_end_pos cannot be trusted, recompute it "the
+        hard way".  */
+      if (!w->window_end_valid)
+       {
+         struct it it;
+         struct text_pos start_pos;
+
+         SET_TEXT_POS_FROM_MARKER (start_pos, w->start);
+         start_display (&it, w, start_pos);
+         move_it_to (&it, -1, it.last_visible_x, window_box_height (w), -1,
+                     MOVE_TO_X | MOVE_TO_Y);
+         window_end_pos = BUF_Z (buf) - IT_CHARPOS (it);
+       }
+
       whole = BUF_ZV (buf) - BUF_BEGV (buf);
       start = marker_position (w->start) - BUF_BEGV (buf);
       /* I don't think this is guaranteed to be right.  For the
         moment, we'll pretend it is.  */
-      end = BUF_Z (buf) - w->window_end_pos - BUF_BEGV (buf);
+      end = BUF_Z (buf) - window_end_pos - BUF_BEGV (buf);
 
       if (end < start)
        end = start;