The fix from Florian in bug 1970802 (to `BrowserTestUtils` to wait for a reflow in the parent process which should cause updated layout info to be posted to the child) I think will get rid of most of the remainder of the failures here? Though not sure how best to check that that is the case.
Bug 1478596 Comment 19 Edit History
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The fix from Florian in bug 1970802 (to `BrowserTestUtils` to wait for a reflow in the parent process which should cause updated layout info for the viewport to be posted to the child) I think will get rid of most of the remainder of the failures here? Though not sure how best to check that that is the case.