So I guess this is part-caused by bug 1899336 - but even without that patch the tab doesn't look right. Instead of the blank space in the example from comment 0 it will show "data..." faded out. It should show the default favicon instead (like pinned tabs in the horizontal tabstrip).
Bug 1909843 Comment 1 Edit History
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So I guess this is part-caused by me applying the in-progress patch from bug 1899336 - but even without that patch the tab doesn't look right. Instead of the blank space in the example from comment 0 it will show "data..." faded out. It should show the default favicon instead (like pinned tabs in the horizontal tabstrip).