When content-visibility:hidden starts skipping a subtree, clear the
stale layout and paint nodes for its descendants.
Preserve SVG mask, clipPath, and pattern resource boxes only when
they are attached to a referencing layout subtree outside the subtree
being hidden. Resources used inside the hidden subtree are cleared
with that subtree, so they rebuild under the live referencing layout
node when the subtree becomes visible again.
Otherwise observed descendants can keep old paintables around after
the current paint tree has been rebuilt without them.
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