a67155e52a
The subdocument call when there's no root frame is silly, because we won't find a canvas frame and we'll end up with transparent. This needs to tweak the canvas frame construction code to make canvas anon content show up on the "top" canvas, not on the page content canvas. This can be simplified now, because the reason we had to build canvas anon content before processing kids of the root frame (bug 1558352) no longer applies since I made popups regular out-of-flow elements (bug 1799343). Differential Revision: https://phabricator.services.mozilla.com/D179461
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