We had two issues with ::backdrop which this commit fixes:
::backdrop is unique in that it's the previous sibling to its
originating element, instead of a child of it. This means when that
element's layout node is thrown away, the ::backdrop's is not.
A second issue is that if we do a partial layout rebuild, the
originating element's layout node replaces its previous one, but we
would still append a new layout node for ::backdrop to the root, so it
would appear in front of the originating element.
A related issue is that clear_pseudo_element_nodes() got called on the
element after its ::backdrop had been assigned, so it would immediately
lose track of it again.
To solve this, we now always remove the ::backdrop's layout node. If we
need to create a new one, we insert it before the element's layout node
if it has one, otherwise we append as before. This ensures we only ever
have up to one layout node for the ::backdrop, and it appears behind
its originating element.
To support this, create_pseudo_element_if_needed() has a couple of
changes:
- It returns the node that was created.
- The caller can ask it not to insert the node, so that the caller can
do so (which we use so that we can insert it in a specific place)
We were always rendering <symbol> SVG elements, but we should only
render them if they are a child of a <use>'s shadow root. This caused
practically all instances of <symbol> to be drawn at least one time too
many.
When an element has `display: contents` and it gets marked for a layout
tree rebuild, we actually have to mark its parent for rebuild as well.
The structure of the parent (and siblings) may change depending on how
the `display: contents` element changes (e.g position, display, etc.)
Otherwise the layout tree will still contain the top layer element(s).
Fixes Steam Events & Announcements `<dialog>` modal visually not fully
disappearing upon removal.
When marking a part of the layout tree for rebuild, if the subtree root
that we're marking has an anonymous parent, we now mark from the nearest
non-anonymous ancestor instead.
This ensures that subtrees inside anonymous wrappers don't just get
duplicated (i.e recreated but inserted instead of replaced).
