The existing code just clipped to a rect. We need to do the same clipping that the non-remote code path does which also handles border radius.
The existing code is the from initial implementation of nsDisplayRemote in https://hg.mozilla.org/mozilla-central/rev/85d06279c8358a8e1c883aa670a20212b1039a90 so I suspect that clipping to the inner view bounds instead of the frame content box is not an important difference.
Differential Revision: https://phabricator.services.mozilla.com/D62180
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extra : moz-landing-system : lando