Minor improvements in vertical cusror motion

* src/xdisp.c (move_it_vertically_backward): Zero out cached value
of line height, to avoid using stale and incorrect values.
(try_window_reusing_current_matrix): Fix conditions for changes in
tab-line height.
Reported by Michael Heerdegen <michael_heerdegen@mailbox.org> in
https://lists.gnu.org/archive/html/help-gnu-emacs/2026-01/msg00163.html
This improves the scrolling a little bit, but doesn't solve the
problem entirely.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index fa826c366dd..22e178fcdc9 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -11300,6 +11300,7 @@ move_it_vertically_backward (struct it *it, int dy)
       int line_height;
 
       RESTORE_IT (&it3, &it3, it3data);
+      last_height = 0;
       y1 = line_bottom_y (&it3);
       line_height = y1 - y0;
       RESTORE_IT (it, it, it2data);
@@ -21673,8 +21674,9 @@ try_window_reusing_current_matrix (struct window *w)
     return false;
 
   /* If top-line visibility has changed, give up.  */
-  if (window_wants_tab_line (w)
-      != MATRIX_TAB_LINE_ROW (w->current_matrix)->mode_line_p)
+  if (!w->current_matrix->header_line_p
+      && (window_wants_tab_line (w)
+          != MATRIX_TAB_LINE_ROW (w->current_matrix)->mode_line_p))
     return false;
 
   /* If top-line visibility has changed, give up.  */