SDL3:Pen:Wayland: SDL does not produce axis (at least pressure) events sometimes

[deiflou]

HI.

I'm using manjaro and I just upgraded the system to Plasma 6.5.4, which dropped support for x11. So I'm using wayland.

I was testing my app on x11 and worked fine, but now on wayland it seems that the pen code does not work always as expected. Sometimes the axis events seem to not be notified. Other times it works fine. I could not say what is going on without checking the code.

In my app I put some logging and the axis events seem to not be sent by sdl, but since my app is a bit complex I tried the pen test, just in case, and it seems to behave the same. I provide a video:

https://www.youtube.com/watch?v=PzX_sWHJiFU

In the video, between each "Pen 3 enters proximity!" and "Pen 3 leaves proximity!" messages, I push the pen on the tablet with different amounts of pressure (the pen down/up events are logged), but only sometimes the square responds to the pressure (and the axis event is not logged).

[Kontrabant]

I'm going to have to punt this to @icculus, as I don't have the hardware to investigate this.

[Kontrabant]

just to narrow this down, would you be able to test in another desktop, such as GNOME or something that uses wlroots like Sway? That would ensure that this is an SDL issue, and not a regression in KDE/kwin.

[deiflou]

I cannot test on another desktop at the moment, sorry.

[icculus]

The fastest way to confirm would be to try something like Krita and see if KDE has the same issue.

(Also, make sure you force SDL to use Wayland, as it likely is using XWayland by default, and that's a totally different chunk of pen code inside SDL.)

[deiflou]

Hi. The other apps work ok. I used SDL_VIDEO_DRIVER=wayland and it behaves the same, but I'm not sure if it uses XWayland since the upgrade, because it dropped X11. I only can start Wayland session now.

I'll check the code when I have time to see if I find something.

[icculus]

Could you also try SDL_VIDEO_DRIVER=x11 (unless KDE doesn't have XWayland support at all?!), and see if that is also broken?

[deiflou]

I also tested the pen in the plasma settings (under wayland the pen tablet settings let you test the pen) and it works fine.

If I use SDL_VIDEO_DRIVER=x11 then it seems to work. It does not report the proximity enter/leave, but the pressure seems to always work. In the following video, between each stream of axis 0 messages I move the pen away and into proximity:

https://www.youtube.com/watch?v=uyKKnNFujQw

[icculus]

Okay, this is probably an SDL bug in our Wayland backend. I'll dig in and see what's up.

[icculus]

I can't reproduce this here, but I'm nowhere near a new enough KDE desktop. I'll spin up Manjaro in a virtual machine when I get a moment.

I think we're going to have to bump this to 3.4.4 for the sake of ever shipping 3.4.2.