bug#42406: Mouse-wheel scrolling can be flickering

[martin rudalics]

> Just to clarify, despite of the title of this bug, it is not a
> flickering, but the mouse-wheel scrolling preformance decreases with
> the number of open frames, in the sense that it becomes more and more
> sticky.

In that case we should consider implementing a variable that updates the
mode lines on one specific frame only.  Does performance not degrade when
you comment out _only_ the two instances of 'wset_update_mode_line' in
'window_scroll_pixel_based'?

martin