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 21:34:53 +0100
> From: Vincent Lefevre <vincent@vinc17.net>
> Cc: 44284@debbugs.gnu.org
> 
> I've found the cause of this issue: with the culprit font, it is
> not always possible to put the text cursor on the last line in
> the window, either with a mouse click or with the down arrow:
> Emacs scrolls the text so that the cursor is at the center of
> the window. Why/when this occurs remains to be determined (is
> there some rounding involved?).

Emacs always requires that the line where point is should be fully
visible.  This is standard Emacs behavior, nothing new and nothing
specific to any special font.

> When the bug does not occur with the mouse wheel, the cursor is
> put on the last line. But with the culprit font, I suppose that
> the above issue causes another scroll in the opposite direction,
> hence the observed behavior.

I suspected that much, but I cannot reproduce this.  When I arrange
for the font to be of size that causes the last line of the window to
be only partially visible, scrolling with the mouse always leaves
point on the line above the last, which is fully visible.  I couldn't
cause the situation you describe, no matter what I tried.

So there's some other factor at work here that somehow forces this
aberrant behavior, and a reproduction recipe is needed to identify
the code which causes it.