Paint order was pure tree order, so a dropdown/overlay declared before a
later sibling lost to it and a z:-1 decorative backdrop covered its own
content. A positioned element with a non-zero z-index now lifts its
whole subtree into a separate paint layer: negative layers under the
normal flow, positive above it, each sorted ascending with tree order
preserved within equal z. Nested z roots paint inside their ancestor
root's subtree in tree order, the pragmatic core of CSS stacking
contexts.
clear (left/right/both, previously unparsed) now ends the float zone,
so a clearfix element drops below the float at full width instead of
joining the flow column beside it.
Repro-verified: an earlier z:10 overlay paints over a later sibling, a
later z:-1 backdrop paints under earlier content, and the clear repro
matches Chromium within 3px on every measure. Spot set unchanged or
slightly improved; obstacle course 33/33.
SGavrl
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