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.
Passing the `AbstractElement` rather than the
`TreeCountingFunctionResolutionContext` allows us to only compute the
resolution context when necessary (i.e. when we actually need to resolve
a tree counting function)
When we compute style for elements inside a UA-internal shadow tree that
represent a pseudo-element (e.g ::placeholder), we actually run the
StyleComputer machinery for (host element :: pseudo-element).
While that lets us match the correct selectors, it was incorrectly
applying CSS inheritance, since we'd also then inherit from whatever was
above the host element in the tree.
This patch fixes the issue by introducing an inheritance override in
AbstractElement and then using that to force inheritance from whatever
is actually directly above in the DOM for these elements instead of
jumping all the way up past the host.
This fixes an issue where `text-align: center` on input type=text
elements would render the main text centered but placeholder text was
still left-aligned.
Tree counting functions should be resolved at style computation time -
to do this we will need to know the element's sibling count and index.
This commit computes that information and propagates it to the various
`StyleValue::to_computed_value` methods.
When a subtree is projected through a slot, its root now inherits style
from the slot's parent, rather than the parent of the unprojected root.
This fixes a ton of subtle issues, and is very noticeable on Reddit.
This reverts 0e3487b9ab.
Back when I made that change, I thought we could make our StyleValue
classes match the typed-om definitions directly. However, they have
different requirements. Typed-om types need to be mutable and GCed,
whereas StyleValues are immutable and ideally wouldn't require a JS VM.
While I was already making such a cataclysmic change, I've moved it into
the StyleValues directory, because it *not* being there has bothered me
for a long time. 😅
...and setter. We had lots of places where we check if pseudo-element
type is specified and then use `pseudo_element_computed_properties()` or
`computed_properties()`. This change moves these checks from caller side
to the getter and setter.
There are multiple things happening here which are interconnected:
- We now use AbstractElement to refer to the source of a counter, which
means we also need to pass that around to compute `content`.
- Give AbstractElement new helper methods that are needed by
CountersSet, so it doesn't have to care whether it's dealing with a
true Element or PseudoElement.
- The CountersSet algorithms now walk the layout tree instead of DOM
tree, so TreeBuilder needs to wait until the layout node exists
before it resolves counters for it.
- Resolve counters when creating a pseudo-element's layout node. We
awkwardly compute the `content` value up to twice: Once to figure out
what kind of node we need to make, and then if it's a string, we do
so again after counters are resolved so we can get the true value of
any `counter()` functions. This will need adjusting in the future but
it works for now.
