Add a document-level boolean flag that tracks whether any :has()
invalidations have been scheduled. This avoids iterating over all
shadow roots just to check is_empty() on each style scope when no
:has() invalidations are pending, which is the common case during
scrolling on complex pages like Reddit.
Results in ~10% reduction of is_empty() calls in profiles when
scrolling on Reddit.
This adds visit_edges(Cell::Visitor&) methods to various helper structs
that contain GC pointers, and makes sure they are called from owning
GC-heap-allocated objects as needed.
These were found by our Clang plugin after expanding its capabilities.
The added rules will be enforced by CI going forward.
Specifically, we create and assign a layer if its import conditions
currently apply.
With this change, every case in the `layer-import.html` test actually
functions correctly, apart from our lack of proper `load` event
support. (Tested by hacking in a 100ms wait after the `await Promise()`
statement.)
Before this change, we've been maintaining various StyleComputer caches
at the document level.
This made sense for old-school documents without shadow trees, since
all the style information was document-wide anyway. However, documents
with many shadow trees ended up suffering since any time you mutated
a style sheet inside a shadow tree, *all* style caches for the entire
document would get invalidated.
This was particularly expensive on Reddit, which has tons of shadow
trees with their own style elements. Every time we'd create one of their
custom elements, we'd invalidate the document-level "rule cache" and
have to rebuild it, taking about ~60ms each time (ouch).
This commit introduces a new object called StyleScope.
Every Document and ShadowRoot has its own StyleScope. Rule caches etc
are moved from StyleComputer to StyleScope.
Rule cache invalidation now happens at StyleScope level. As an example,
rule cache rebuilds now take ~1ms on Reddit instead of ~60ms.
This is largely a mechanical change, moving things around, but there's
one key detail to be aware of: due to the :host selector, which works
across the shadow DOM boundary and reaches from inside a shadow tree out
into the light tree, there are various places where we have to check
both the shadow tree's StyleScope *and* the document-level StyleScope
in order to get all rules that may apply.
