Re: master 5b5608c614: Fix glyph skipping optimization when a tab line is enabled

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Mon, 18 Apr 2022 15:32:42 +0800
> 
> >> diff --git a/src/dispnew.c b/src/dispnew.c
> >> index 0d95904..3cfe1b8 100644
> >> --- a/src/dispnew.c
> >> +++ b/src/dispnew.c
> >> @@ -3928,9 +3928,12 @@ update_text_area (struct window *w, struct 
> >> glyph_row *upd
> >>          However, it causes excessive flickering when mouse is moved
> >>          across the mode line.  Luckily, turning it off for the mode
> >>          line doesn't seem to hurt anything. -- cyd.
> >> -         But it is still needed for the header line. -- kfs.  */
> >> +         But it is still needed for the header line. -- kfs.
> >> +         The header line vpos is 1 if a tab line is enabled.  (18th
> >> +         Apr 2022) */
> >>        || (current_row->mouse_face_p
> >> -         && !(current_row->mode_line_p && vpos > 0))
> >> +         && !(current_row->mode_line_p
> >> +              && (vpos > w->current_matrix->tab_line_p)))
> >>        || current_row->x != desired_row->x)
> >>      {
> >>        output_cursor_to (w, vpos, 0, desired_row->y, desired_row->x);
> >
> > I'm not sure I understand this change.  What exactly is the problem,
> > and how to reproduce it?  What if there's no header-line, but there is
> > a tab-line, or vice versa, or both are displayed?
> 
> Not sure, but the previous code would also trigger for just tab lines,
> but not also the header line if there was a tab line.
> 
> Here is how to reproduce the problem this fixes: turn on
> global-tab-line-mode, open Info, type "m cl RET g Predicates RET", then
> click on the "e" in the "Next: Control Structure" link on the header
> line.  "Next:" will continue to be displayed in the mouse face even
> though the mouse is no longer on any link, due to the "glyph-skipping"
> optimization that is normally disabled for the header and tab lines
> being used.

OK, and what about the last question I asked above?  If there's no
header-line, but there is a tab-line, the code will still compare vpos
with 1, although the vpos of the tab-line is now zero, right?