I have attempted reproducing this issue on Ubuntu 20 with i3 Windows Manager installed.(https://kifarunix.com/install-and-setup-i3-windows-manager-on-ubuntu-20-04/) Fortunately, I managed to reproduce it in the Nightly v90.0a1 and Beta v89.0b4-v89.0b14, but it would not reproduce in the same in Firefox Release v88.0.1, so it is a regression. It does not reproduce in Ubuntu with default Windows Manager, only with i3 Windows Manaager, where the new Global Sharing Overlay UI is improperly be displayed.
Bug 1705721 Comment 12 Edit History
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I have attempted reproducing this issue on Ubuntu 20 with i3 Windows Manager installed.(https://kifarunix.com/install-and-setup-i3-windows-manager-on-ubuntu-20-04/) Fortunately, I managed to reproduce it in the Nightly v90.0a1 and Beta v89.0b4-v89.0b14, but it would not reproduce in the same in Firefox Release v88.0.1, so it is a regression. It does not reproduce in Ubuntu with default Windows Manager, only with i3 Windows Manaager, where the new Global Sharing Overlay UI is improperly displayed.