Key processing delayed

[jengelh]

Version: sdl2-compat-2.32.56 + SDL 3.2.16
OS: openSUSE Tumbleweed 20250610 amd64; Linux 6.15.1 amd64; Xorg 21.1.15; xfce 4.20.2; libinput 1.28.1

Affected games:

• https://github.com/fabiangreffrath/taradino as of June 17 2025
• https://github.com/coelckers/prboom-plus
• https://github.com/kraflab/dsda-doom before a65765d474e0605402a314be7519bd4720e55d01

What's not affected:

• games using SDL3 directly
• same game binaries, unmodified, using "non-compat" libSDL-2.0.so.0 dropped back in

To trigger:

• Press strafe left key on keyboard. Do this for at least 5 seconds; or longer if needed to reach a wall/obstacle that zeroes the player's movement speed.
• Let go and press strafe right

Observed behavior:

• Strafe right takes effect with a delay

Expected behavior:

• Strafe right should take immediate effect

cc @fabiangreffrath

[slouken]

Are you still seeing this with the latest SDL?

It looks like the dsda-doom fix was to turn off text input, but that was enabled by default in SDL2, so I'm not sure what's different here.

Out of curiosity, if you're still seeing this, what keyboard layout are you using? Do you still see the issue if you switch to en-US keyboard layout?

[jengelh]

Yes this is still present with SDL 3.2.22/sdl2-compat 2.32.56 (/ ibus-1.5.32).

Keyboard layout is en_US already. If I place the movement actions on non-lettered keys like Ctrl and Shift, I experience no lag. That's interesting. If I turn off ibus altogether, the lag when using lettered keys also goes away.

Still, pure SDL2 even with ibus being active (with en_US) is fine, so that's that.

[slouken]

If you run ibus-setup, what input methods do you have installed?

I double-checked here, and I'm running prboom-plus with the IBus IME and no input methods installed, and WASD inputs take effect immediately.

[jengelh]

[jengelh]

I am using XFCE 4.20.2 as DE and found that, when going into xfce4-settings-manager and disabling the section about key repeat (pictured), the lag problem also goes away.

Moreover, the lagginess is proprtional to the chosen repeat speed number. Pick '1', prboom works ok, pick 80 and it takes a really long time to react. (xset r rate 250 80 has the same effect, so it does not look like it needs xfce4. Running prboom with $XMODIFIERS unset also makes the issue go away. So ibus definitely plays some role here.)

[slouken]

Ah, that makes sense. It sounds like ibus is holding onto the key event to see whether or not to repeat it.

[jengelh]

Reverting to ibus 1.5.28 also appears to "fix" it. Given that, it starts to smell like ibus/ibus#2618

[slouken]

Ah, yes, that seems likely. Thanks for the research on this!

[jengelh]

7502abbe7eb4289d0d727e53cdfac572baf405d5 is the first bad commit.

Starting (modern) ibus-daemon-1.5.32 as IBUS_ENABLE_SYNC_MODE=2 ibus-daemon -rvx "fixes" it.
Still wondering though how SDL2 differs from sdl2-compat now that we know it's related to ... those sync modes.