The contain:paint clipping would be redundant and hence unnecessary in this
scenario, because:
- Scroll frames already clip their descendant frames.
- contain:paint has other (non-clipping-related) effects that prevent
descendant frames from escaping the scrollframe ancestor.
So, no further clipping is required.
This is a behavior change - it works around an issue that makes us fail to
repaint mousewheel-scrolled content inside of any scrollframe that returns true
from ShouldApplyOverflowClipping().
Differential Revision: https://phabricator.services.mozilla.com/D12056
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extra : moz-landing-system : lando
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