`SlottableMixin::assigned_slot()` was using `as<DOM::Node>(*this)` to
get a `Node` reference. Since `SlottableMixin` has no inheritance
relationship with `Node`, `as_if<>` can't use `static_cast` and falls
through to `dynamic_cast`, which is expensive. Replace this with a
virtual `slottable_as_node()` accessor overridden in `Element` and
`Text`.
This showed up as hot in profiles when loading the GC heap explorer
page.
This change introduces SlotRegistry to track slot elements per shadow
root. This allows us to iterate slots directly when assigning
slottables for a tree instead of walking an entire subtree.
The microtask is conceptually global and fires pending observers at the
agent level. As such, it doesn't make sense for it to be associated with
any specific document.
This would happen for example when removing a slot element from the DOM,
in which case it would keep its old list of assigned slottables even
though it now has none. Fixes a couple of WPT tests.
We use a Vector for this, but its spec definition is an ordered set.
That means we need to ensure we don't add duplicates. This fixes issues
where we would send slotchange events multiple times to the same
HTMLSlotElement.
Returning a Vector of Slottable is not very nice here, but this
matches find_slottable (which this calls), and as far as I can
tell this is technically 'safe' at the moment in the way in
which it is / will be called.
It's also not great that like find_slottable it takes a non-const
ref, but changing that causes a bunch of other fallout.
To allow for adding the concept of a WorkerAgent to be reused
between shared and dedicated workers. An event loop is the
commonality between the different agent types, though, there
are some differences between those event loops which we customize
on the construction of the HTML::EventLoop.
We achieve this by keeping track of the number of HTMLSlotElements
inside each ShadowRoot (do via ad-hoc insertion and removal steps.)
This allows slottables assignment to skip over entire shadow roots when
we know they have no slots anyway.
Massive speedup on https://wpt.fyi/ which no longer takes minutes/hours
to load, but instead a "mere" 19 seconds. :^)
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
