See https://treeherder.mozilla.org/#/jobs?repo=graphics&fromchange=cbd6439cd06f06771c3a0fa443899c4bdf88e928&group_state=expanded&bugfiler&filter-searchStr=quantum%20media for example - all (or almost all) of the mda1 failures are in the same thing. Looking at the log, we see that the shutdown watchdog is firing, because thread 0 (main thread) is stuck in PCompositorBridgeChild::SendWillClose. If we look at the compositor thread, it's stuck in a call to WebRenderAPI::WaitFlushed which is called from WebRenderBridgeParent::ClearResources(). That in turn is blocked on the render thread, which is busy doing RenderThread::UpdateAndRender (at least in the case I looked at ) and seems to have a kind-of-garbage stack. The stack seems to claim it's stuck in a SwapBuffers call but I'm not sure if I trust that.  https://treeherder.mozilla.org/logviewer.html#?job_id=97360211&repo=graphics&lineNumber=2641
Optimistically closing as fixed by bug 1365196, will reopen if they come back.s
Status: NEW → RESOLVED
Last Resolved: 9 months ago
Resolution: --- → FIXED
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