bug#72323: 31.0.50; line-move unconditionally resets vscroll to 0

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

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Steven Allen <steven@stebalien.com>
>> Cc: luangruo@yahoo.com, 72323@debbugs.gnu.org, storm@cua.dk
>> Date: Sun, 18 Aug 2024 10:38:29 -0700
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> > Once again: the vscroll value is pertinent only for the screen line
>> > for which it was computed, because the way it is computed uses the
>> > metrics of that line.  Once you move to another line, the value is no
>> > longer pertinent.
>> 
>> Let's be precise about "move to another line":
>> 
>> - When window-start changes, vscroll becomes invalid because the lines at
>>   the top of the screen (to which vscroll applies) has changed. I agree
>>   that it must be reset in this case.
>> - When point is moved up and down in such a way that window-start isn't
>>   changed, vscroll is still perfectly valid as the top line in the
>>   window hasn't changed.
>
> I disagree.  When point moves to another screen line without moving
> window-start, vscroll may or may not be valid.  Whether it is depends
> on the details of what is on display.

Can you give me an example example where moving point invalidates
vscroll (except when point would move partially or fully off screen)?