bug#57728: 29.0.50; Emacs writes wrong glyph at the bottom-right corner of text terminals

[Gerd Möllmann]

Gregory Heytings <gregory@heytings.org> writes:

>> But AFAIU, the bug actually shows that we somehow _do_ write there,
>> although the display engine assumes we don't.  Right?
>
> Yes, that's what the bug shows.  We do (sometimes) write something
> there, and we shouldn't.  (And if the feature request in bug#57607 is
> implemented, we should write something there, but always.)

Perhaps insert_glyphs shifts something into the corner?  Or clean_to_end
doesn't clear?  If that's the case, then write_glyphs not writing to the
corner would be the culprit.