[mtl] fence blocks indefinitely upon switching to fullscreen

[ghost]

Waiting for the fence passed to submit_without_semaphores blocks indefinitely when switching from the fullscreen space to desktop and then back again. The command buffer seems to remain in MTLCommandBufferStatusCommitted forever. Specifying a large timeout for wait_for_fence (such as one second) can be used to work around the issue, but will result in a noticeable jitter when switching back to the application.

[kvark]

Thank you for filing this!
Any idea on what happens there that makes the command buffer stuck in MTLCommandBufferStatusCommitted?

[ghost]

I was half-expecting there to be a status code that we weren't checking for in wait_for_fence (such as MTLCommandBufferStatusError), but this doesn't appear to be the case. My best guess is that a command buffer gets committed for rendering, but never gets rendered because the window is no longer visible. I'm not sure if this is intended behaviour in Metal, and if there's something we're expected to be doing.