bug#44284: 27.1; with some Unicode font, scrolling upward with the mouse wheel actually scrolls downward when the cursor needs repositioning

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 30 Oct 2020 21:57:02 +0100
> From: Vincent Lefevre <vincent@vinc17.net>
> Cc: 44284@debbugs.gnu.org
> 
> This is contextual. For instance, with
> 
>   emacs -Q -fn -misc-fixed-medium-r-semicondensed--13-120-75-75-c-60-iso10646-1 file
> 
> I have a window with 34 lines:
> 
> 1
> 2
> 3
> .
> .
> .
> 33
> 34
> 
> If I move the cursor downward with the down arrow, I can reach
> line 33. At line 34 (last line of the window), Emacs scrolls to
> have:
> 
> 17
> 18
> 19
> .
> .
> .
> 49
> 50
> 
> with the cursor on line 34. Then, with the down arrow, I can put the
> cursor on line 50, which remains the last line of the window. With
> the up arrow, I put the cursor on line 40. With the mouse wheel, I
> scroll downward (one time), then upward. I still have lines 17 to 50
> with the cursor on line 40. With the down arrow, I can reach line 49,
> but at line 50, Emacs scrolls to have lines 33 to 66, while before,
> the cursor could be on line 50 as the last line of the window.

Like I said, there's some factor at work here that needs to be
identified.  Which is why I need a reproduction recipe.

Alternatively, someone else who can reproduce this are welcome to
debug this and propose analysis and/or fix.

Thanks.