[arch-general] Keyboard lag issue

[Manuel Groß]

Hello there,

I currently experience a weird keyboard lag issue which I yet failed
to resolve. I had some help looking into it a bit (and obviously
searched the web), but we couldn't identify the speficic problem. I hope
someone could provide me pointers or hints on what to test out.

I have a Thinkpad x250 running Arch Linux with Openbox and X11 on it.
I use systemd-logind to log in, so X is started with the 'exec
openbox-session' from my xinitrc. Not from some fancy lightdm or so.
I do not have a certain date since when the issue occurs, but it has
probably been a little over a month. I didn't make any major changes to
my system then, and I do update regularly. This didn't occur when I set
up the system.

The issue is, that on some, certain programs, the keyboard input
'arrives' too late, with delays up to 1-2s. In other programs, this
never happens at all. And in rare occasions, it is even fine for
programs, in which I experience the problem.
I checked logs at obvious locations, but neither the systemd journal,
dmesg or the Xorg log were screaming 'I have an issue here'.
The trackpoint always works as excpected and the touchpad is disabled.

Programs where the delay occurs:
 * sakura
 * sublime
 * wireshark (qt5 version)

Programs which are fine:
 * claws-mail
 * geany
 * xterm
 * chromium

The delay also never occurs in the system shell (Alt+F2) and xterm, so
it's not the shell or oh-my-zsh.
The program lists suggests, that it is not a problem due to a specific
version of GTK or QT, since programs using either library (or none of
them) are affected.

I tried using 'libinput' instead of 'evdev' as a keyboard driver,
but the change had no effect (I double checked the Xorg log which driver
was actually used).
In 'xev', all keystrokes arrive immediately.

So seems not to be the kernel or the driver, nor a specific gtk or qt
library.
The next thing to test would probably be the xlib and the libxcb
themselves. So I used 'downgrader' to downgrade each a few revisions
(also their lib32-libraries) and checked for a change, without success.

I don't know what to try next. Does anyone of you have an idea or
already solved a similar issue? I'd appreciate any help on this.
-- 
Grüße/Regards
Manuel Groß
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