I guess the issue is because the mouse moves too fast and makes it to be outside the browser before the content process tries to reset its position. After bug 1662587, now the content process will send an IPC to the parent to request a pointer lock, and maybe we could set pinter capture in the parent process while it knows which remote target requests a pointer lock.
Bug 853160 Comment 9 Edit History
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I guess the issue is because the mouse moves too fast and makes it to be outside the browser before the content process tries to reset its position. After bug 1662587, now the content process will send an IPC to the parent to request a pointer lock, and maybe we could set pinter capture in the parent process while it knows which remote target requests a pointer lock?