bug#70038: 29.3.50; Shift up/down in buffer with images on M-x other-window with some fonts

[martin rudalics via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

 >> There are three issues I don't understand well in this context.  The
 >> first one concerns the preserve_vscroll_p flag we currently check as:
 >>
 >>        if (!w->preserve_vscroll_p && !window_frozen_p (w))
 >> 	w->vscroll = 0;
 >>
 >> Here, in all scenarios I tried, I observed that w->preserve_vscroll_p
 >
 > ???  Have you enabled pixel-scroll-precision-mode, for which this flag
 > was introduced?

Yes.

 > p-s-p-m relies on vscroll values that it sets being
 > preserved by force_start in all scenarios, which naturally includes
 > those where the window start is frozen, just as it does those where it
 > is variable.

When I ran the above with various debugger breakpoints I was not able to
trigger a configuration with w->preserve_vscroll_p true on an unfrozen
window.  In all the scenarios I tried, w->preserve_vscroll_p was reset
in mark_window_display_accurate_1 before arriving there.  Can you please
post a scenario where the difference matters?  If so, it might be worth
to shortly document it somewhere.

Thanks, martin