bug#42406: Mouse-wheel scrolling can be flickering

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: alan@idiocy.org,  konrad.podczeck@univie.ac.at,  42406@debbugs.gnu.org
> Date: Thu, 17 Dec 2020 16:24:01 -0500
> 
> > And the window's update_mode_line flag should not be used for deciding
> > about all the -lines and frame-title, we should instead have much more
> > fine-grained flags that would allow us to make separate decisions
> > regarding each one of them;
> 
> That would be nice, yes.  But that's largely independent from the bug
> under discussion, where the performance seems to be adequate with the
> current coarse grained flags as long as we have a single frame, so the
> problem seems to be that we consider all frames instead of only
> considering those displaying the affected windows.

No, the problem is that we turn on update_mode_lines because we need
to make sure the title of the _selected_ frame, a single frame, is
updated due to possible effects of scrolling on that single title.
And the effect is to consider the titles (and menu bars and tool bars)
on all frames instead, which is definitely unnecessary.