bug#12401: scroll-preserve-screen-position broken with font height faces

[Stefan Monnier]

>> > scroll-preserve-screen-position?  Is it just that doing N C-v's
>> > followed by N M-v's brings point to the same location where it was
>> > before this sequence of 2N commands?
>> I think this should be the main point, yes.
> For this to work, we would need to make sure that C-v followed by M-v
> end up with a window that has the same window-start as the one before
> C-v.  But this is impossible to ensure with the way we compute
> window-start, because the move_it family of functions used for that
> can only move forward; therefore moving back is necessarily
> implemented differently, and the results differ when variable-size
> fonts are present.

Indeed.

> IOW, this is a limitation of the current design of the
> window-scrolling functions that can only be lifted by redesigning.

Yup,


        Stefan