If a CSS animation or transition was being used to manipulate a property
that itself does not affect layout, we were still doing a full relayout
whenever any animation or transition related property was changed.
As it turns out, we can just not do that, and we avoid a bunch of
unnecessary layout work on many pages. When a layout-affecting property
is being animated, the animation/transition update code takes care to
invalidate layout as appropriate anyway!
This was very noticeable on GitHub, where moving the mouse cursor
between "Issues" and "Pull requests" would trigger a full relayout every
time. Now that doesn't happen, and it's much more responsive. :^)
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