GNU bug report logs - #59265
27.1; emacs apparently redraws the entire X window for every update

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From: Po Lu <luangruo <at> yahoo.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 59265 <at> debbugs.gnu.org, Francesco Potortì <pot <at> gnu.org>
Subject: bug#59265: 27.1; emacs apparently redraws the entire X window for every update
Date: Tue, 15 Nov 2022 21:57:23 +0800

Eli Zaretskii <eliz <at> gnu.org> writes:

> One thing I can suggest to try is disable double-buffering.  I have no
> idea what it can do with Xpra (since I don't really know how does Xpra
> do its job, whatever that is).

Yes, I explained what the problem is (and it isn't in Emacs) earlier:

Judging by the debug info provided in the Xpra bug report, it is relying
on the Damage extension to report window damage.  The Damage extension
always reports damage to the entire window upon a DBE buffer swap, which
is almost inconsequential when connected to an actual remote X server (I
do that every day), but not when a program like Xpra tries to ferry
pixmap contents over the wire.

Relying on Composite and Damage to send application contents over the
wire is fundamentally the wrong approach.  I suspect Xpra has to be
written as an X server driver (as its name applies), and that after
doing so, its DBE implementation has to be fixed to only call the
CopyArea GC op on areas of the back buffer that really changed.

So this isn't really a bug in Emacs, sorry.  You can work around the
problem by turning off double buffering, but that means you will also
need to put up with flicker.

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