Affected versions

• Firefox 80.0b5
• Firefox 81.0a1

Affected platforms

• macOS 11 (public beta 1)

Steps to reproduce

1. Launch Firefox
2. Access any video on youtube
3. Drag Firefox window on the secondary display then drag it back to the primary display

Expected result

• The page has the same size

Actual result

• The page shrinks for a second or two then stretches back to it's original size

Regression range
I don't think this is a regression.

Additional notes

• The issue can be seen in the following attachment.
• Dual display is needed in order to reproduce this issue
• It happens on all websites, but if there is a video rendering on the page (ex. youtube), the shrinked page will stay longer until it stretches back to the original size.

The severity field is not set for this bug.
:alaskanemily, could you have a look please?

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Probably more of a Widget-level issue than Layout.

This looks like it might require a fix similar to bug 1154125, which was in Layout. Is it possible that bug 1154125 has regressed? Happy to take it back in widget:cocoa if this is found to be a widget-level issue now.

Bug 1154125 was different, in that (IIRC) the incorrect scaling was persistent until the window was manually resized (or something else triggered a re-layout), because we'd used the wrong scale factor. Here, the final state is fine, it's just an ugly transitional issue. Seems like it's probably related to how we handle resolution-change notifications from the OS; maybe there's a change in 11.0 in how/when these are dispatched or something.

Ah, thanks. We have had reports of this before, but I can't seem to find the right bug to duplicate this to at the moment. I will take another look.