We were failing to discriminate between DOM removals happening to SVG
elements cloned as part of an SVG use element instantiation.
When a "use source" element is removed, all clones of that source must
be updated to reflect the change. But when a "use clone" element is
removed, that's fine.
This was causing the surprising disappearance of use element subtrees,
seen for example on https://cal.com/
Since SVG gradients can reference each other, we have to keep track of
visited gradients when traversing the link chain, or we will recurse
infinitely when there's a reference cycle.
This attribute has some compatbility issues...
- The spec says it should be an SVGAnimatedRect which contains
a DOMRect and a DOMReadOnlyRect.
- Blink gives you an SVGAnimatedRect with 2x SVGRect
- Gecko gives you an SVGAnimatedRect with 2x SVGRect? (nullable)
I ended up with something similar to Gecko, an SVGAnimatedRect
with 2x DOMRect? (nullable)
With this fixed, we can now load https://polar.sh/ :^)
It does not currently handle any of the actual scripting, but this
should at least allow us to create an instance of the element.
The test being added here isn't actually testing much, but before the
previous commit we used to crash parsing the page due to a TODO().
