Expose the paintable tree dump through the internals object, analogous
to the existing dumpLayoutTree(). This is useful for testing and
debugging painting behavior.
This new testing API dumps the layout subtree rooted at a given DOM
node. It will be useful for testing partial layout tree rebuilds, where
we need to verify the layout tree structure for specific subtrees rather
than the entire document.
This test verifies that stacking contexts are correctly created and
removed when CSS properties that affect stacking context are changed:
- opacity: transitioning to/from 1
- transform: transitioning to/from none
- filter: transitioning to/from none
- isolation: transitioning to/from isolate
It also verifies that changes within the "creating" range (e.g., opacity
0.5 -> 0.3, or transform translateX(10px) -> translateX(20px)) correctly
preserve the stacking context.
This lets you access closed shadow roots from JavaScript, even though
they're not normally accessible to JavaScript. This can be used to poke
into UA shadow roots in tests.
