Do not let elements inside display:none subtrees become focus targets.
The HTML focusable-area model only allows elements to be focusable when
they are rendered, delegate rendering to their children, or are relevant
canvas fallback content.
Preserve blur for the current focused area after script hides it, since
the unfocusing steps operate on the old focus target. Check display:none
through the flat-tree style parent chain, so slotted controls inside
hidden slot subtrees cannot become focused.
Cover hidden ancestors with materialized computed style, display:none
controls, display:contents, hidden focused controls, and slotted cases
inside hidden and visible shadow-tree subtrees.
Prevents observably calling Trusted Types, which can run arbitrary JS,
cause crashes due to use of MUST and allow arbitrary JS to modify
internal elements.
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
