The viewport size could be changed because [we don't allow __minimum-scale size__ on fullscreen state](https://searchfox.org/mozilla-central/rev/6cc48251bb97600fdf11a5b4c5f621bfc8606d55/layout/generic/nsGfxScrollFrame.cpp#6194-6200), I tried to drop the restriction in bug 1650686, but it's not yet happened. Anyways, the restriction is only affecting where the minimum-scale size is applied to the document, is the document having an element bigger then the initial containing block size? That said, even if the viewport size is changed, I don't know how it's affecting the video size at the first glance of the code in comment 4. Probably the best way to move this issue forward to find out the regression range.
Bug 1659761 Comment 5 Edit History
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The viewport size could be changed because [we don't allow __minimum-scale size__ on fullscreen state](https://searchfox.org/mozilla-central/rev/6cc48251bb97600fdf11a5b4c5f621bfc8606d55/layout/generic/nsGfxScrollFrame.cpp#6194-6200), I tried to drop the restriction in bug 1650686, but it's not yet happened. Anyways, the restriction is only affecting where the minimum-scale size is applied to the document, is the document having an element bigger than the initial containing block size? That said, even if the viewport size is changed, I don't know how it's affecting the video size at the first glance of the code in comment 4. Probably the best way to move this issue forward to find out the regression range.