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

[Vincent Lefevre]

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?).

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.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)