bug#72960: 31.0.50; PGTK Wayland exhibits more lag than X11 version

["Stephane Travostino" <sph@combo.cc>]

On Mon, 2 Sep 2024, at 13:05, Eli Zaretskii wrote:
>> Date: Mon, 02 Sep 2024 10:18:03 +0100
>> From: "Stephane Travostino" <sph@combo.cc>
>> 
>> Heavy operations, such as scrolling back and forth in a buffer, are
>> noticeably laggier, for lack of better word, in the PGTK/Wayland version
>> than the X11, both tested on KDE in Wayland mode. 
>> 
>> Affects both 29.2 and the latest HEAD compiled a few days ago.
>> 
>> I am unsure whether it is a KDE or Emacs problem.
>> 
>> Running on an AMD RX 6800 XT graphics card on a HiDPI 4k screen at 2x
>> scaling.
>
> AFAIU, this is a problem with GTK input methods.  From PROBLEMS:
>
>   *** Emacs built with GTK lags in its response to keyboard input.
>   This can happen when input methods are used.  It happens because Emacs
>   behaves in an unconventional way with respect to GTK input methods: it
>   registers to receive keyboard input as unprocessed key events with
>   metadata (as opposed to receiving them as text strings).  Most GTK
>   programs use the latter approach, so some modern input methods have
>   bugs and misbehave when faced with the way Emacs does it.
>
>   A workaround is to set GTK_IM_MODULE=none in the environment, or maybe
>   find a different input method without these problems.

Thank you, though without more scientific methods of measuring latency I can't tell if that helps or not. 

I noticed I had pixel precision scrolling mode on and that contributed a large part to that feeling of lag compared to other programs. If Firefox is able to smooth scroll at 60 Hz, I would say empirically Emacs PGTK would scroll at 15 Hz, making navigation in the buffer a choppy affair.