Aliaksandr Kalenik 36acd1cf3d LibWeb: Rebuild stacking context tree on any z-index value change ...

The stacking context tree rebuild had an optimization that skipped
rebuilding when a property changed between two values that both create
stacking contexts. This is correct for most properties (e.g. opacity
0.5 -> 0.8 doesn't change tree structure), but incorrect for z-index.

During tree construction, elements with z-index 0/auto are placed in
m_positioned_descendants_and_stacking_contexts_with_stack_level_0,
while elements with other z-index values are painted from m_children
(negative at step 3, positive at step 9 per CSS 2.1 Appendix E).
When z-index changed between non-auto values (e.g. 0 -> 10), the
optimization skipped the rebuild, leaving the element in the wrong
list and causing it to be painted from both step 8 and step 9.

This was visible on pages like shopify.com where elements with
transition-all would transition z-index, producing a flood of
"Painting commands are recorded twice for stacking context" messages.

2026-02-17 20:44:46 +01:00

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