bug#42406: Mouse-wheel scrolling can be flickering

[Stefan Monnier]

>> But in the present case, the only change is the w->start
>> an the position of point: these are important enough to need to update
>> the mode line (for the column&line number), but I don't think these are
>> important enough to justify setting `update_mode_lines` to 42:
>> not only other frames can't be affected, but even the current frame's
>> title should basically never be affected either because it would be very
>> unusual to have the line/column number as part of the frame title.
>
> "Unusual" doesn't mean "unsupported".

I just tried to test my proposed patch to see if it breaks this feature,
and it turns out that it's very definitely unusual: I don't know if it's
supposed to be supported, but at least I can say that it doesn't work ;-)

At least:

    emacs -Q --eval '(setq frame-title-format "(%l,%c)")'

shows me "(,)" as the frame title with the code in `master` as well as
with Emacs-25 and Emacs-27.

So maybe the better approach to fix this performance problem is indeed
not with the patch I proposed (which might still be a good idea in
general) but with some other change which refrains from setting
`update_mode_lines` at all just because we're scrolling.


        Stefan