Gfx: Selecting either Wayland or X11 for the GL backend, when both are available

[torokati44]

Short info header:

• GFX version: 0244e3401e9f127617cb8636397048584e7bfe8a
• OS: Fedora 33 (Linux 5.9.15-200.fc33.x86_64), KDE Plasma 5.20.4, Wayland session
• GPU: Intel HD Graphics 4000 + AMD Radeon HD 7730M

I'd like to give the newly merged GL backend a try, with Ruffle (with a 2-line patch on top of its master, to allow selecting the GL backend).
On startup, I get:

[... INFO gfx_backend_gl::window::egl] Using X11 platform

But then:

[... INFO wgpu_core::instance] Instance::create_surface
thread 'main' panicked at 'Unsupported window: Wayland(WaylandHandle { surface:

I tried forcing gfx to either use only X11 (through XWayland), _or_ only Wayland (natively); but could only do the former, by setting WAYLAND_DISPLAY=''
The same method doesn't work for disabling X11 though (DISPLAY=''), gfx will still try to use X11, but will fail to open the display.
I also tried EGL_PLATFORM=wayland, but that didn't seem to do anything.

[kvark]

I imagine that we could prefer Wayland if we discover it to be available, over X11, and expect all windows to be Wayland handles.

[torokati44]

That seems reasonable to me.
Will the WAYLAND_DISPLAY='' forced fallback to X11/XWayland still work then, with the added expectation?

[kvark]

No idea. We need to find out!

[nemosupremo]

Commenting because I worked in this recently. For the GL backend I recently added support for wayland (#3545), before that it only supported X (which is the version you are using).

I think the issue you are hitting is actually because of winit - winit by default will prefer to use wayland directly and gives us a wayland window if possible - which obviously causes problems as gfx had only supported X. I’m sure you can force winit to use XWayland which should make things work in your case.

I don’t know if gfx should attempt a fallback here - the issue (which I hit) is that gfx attempts to create a context before we know for use what window manager we will be drawing into. If they don’t match, then the context has to be recreates

[torokati44]

I’m sure you can force winit to use XWayland which should make things work in your case.

Yes, I could, and I mentioned this in the original report as well.
It's just... How does this work at all for anyone on Wayland (directly) who _also_ has X11 available (via XWayland)? This will be the case for a vast majority of users for many years still, I believe.

[nemosupremo]

Right, the way it currently works on master is it first checks for a wayland session, and if that fails it falls back to X. This previously didn’t work as the GL backend didn’t support Wayland.

[torokati44]

Ah, I see. Wasn't aware that this was already changed in master, sorry.