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

[Vincent Lefevre <vincent@vinc17.net>]

On 2020-10-30 21:34:53 +0100, Vincent Lefevre wrote:
> 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?).

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.

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