In particular it could be that the `ShutdownConfirmedHP` message is not processed if there are already many events in the `control` priority queue. Looking at [where we use `control`](https://searchfox.org/mozilla-central/search?q=%5BPriority%3Dcontrol%5D&path=&case=false®exp=false), to me the usage in Graphics: Layers (compositor) stands out, in particular we did bug 1730998.
Bug 1755376 Comment 18 Edit History
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In particular it could be that even the `ShutdownConfirmedHP` message is not processed if there are already many events in the `control` priority queue. Looking at [where we use `control`](https://searchfox.org/mozilla-central/search?q=%5BPriority%3Dcontrol%5D&path=&case=false®exp=false), to me the usage in Graphics: Layers (compositor) stands out, in particular we did bug 1730998.