Re: How to debug emacs repaint problems?

[Eli Zaretskii <eliz@gnu.org>]

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> From: joakim@verona.se
> Cc: emacs-devel@gnu.org
> Date: Sat, 14 Oct 2023 22:06:31 +0200
> 
> I managed to make a screenshot that shows the problem. In the image I
> made an emacs window with some black text on red background.
> 
> If I move the cursor just one line, you can see that 2 large regions
> become blurred. Since I just moved the cursor one line down, its rather
> confusing why these 2 seemingly unrelated regions should be redrawn.

It is not Emacs that's doing that.  When you move the cursor, Emacs
redraws just two characters: the one where the cursor was before the
move, and the one where it is after the move.  It can also redraw a
small part of the mode line: if the cursor is moved vertically, the
part with the line number will be redrawn, and if you have
column-number-mode enabled, the column number will be redrawn for any
cursor movement.  That's all.

I think Po Lu explained why you see what you see.  If disabling
double-buffering doesn't help, I think the root cause is the VNC
software you use, which must somehow decide which portions of the
screen to redraw.  Emacs is not the culprit here.