In WindowsPreviewPerTab, requestPreview calls resetCanvasPreview and then updateCanvasPreview.
https://searchfox.org/mozilla-central/rev/a92ed79b0bc746159fc31af1586adbfa9e45e264/browser/modules/WindowsPreviewPerTab.jsm#294-295
The former sets the preview canvas' size to 0x0. In updateCanvasPreview, we then ask to captureToCanvas,
and we end up in _captureRemoteThumbnail, which uses the passed aWidth and aHeight to size the destination
canvas. Unfortunately, those are 0. We should ignore this size when the fullScale argument is passed, and
in that case just use the content width and height directly instead.
Differential Revision: https://phabricator.services.mozilla.com/D59060
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extra : moz-landing-system : lando
b4664fb355