…le DOM. (...)

This new implementation is based around the assumption that all of the features
enabled by searchable invisible DOM require those elements to actually be in
the main document.

…dren] now handles inserting children.

…r. (...)

ChildManager doesn't serve any purpose: MutationObserver is needed to discover
parser-inserted children and MutationObserver callbacks are called before the
next paint, giving overridden Node methods no performance advantage.

…move `top` cache.

…e children.

…height estimate error.

…ed again later.

…eight estimates from being set to 0.

…sing scroll event targets. (...)

The target of scroll events isn't always the element that needs to have its
`scrollTop` corrected when height estimates are found to be incorrect during an
update. This can happen if the virtual content is nested in multiple scrollable
elements and the user scrolls more of the virtual content into view using one
of the outer scrollable elements.

…th scrolling interruption. (...)

https://bugzilla.mozilla.org/show_bug.cgi?id=1502059

…with the viewport.

…rt intersection with unrendered space. (...)

Before this change, the virtual-content did not update if its nearest scroll
container (possibly the window) resized in a way that made more of the
virtual-content visible. To fix this, sentinel elements sized to fill the space
between visible elements are inserted and monitored with an
IntersectionObserver, using the viewport as the root. This has the convenient
side effect of covering all of the cases that were previously handled by
listening to scroll events. Additionally, this prevents any virtual-content
elements that do not intersect the viewport from updating for scroll events.

…tent's viewport intersection state is unnecessary. (...)

Tracking the state of the virtual-content element's intersection with the
viewport isn't necessary because the callback won't be called if the
virtual-content element's intersection with the viewport didn't change or the
virtual-content element doesn't intersect the viewport (meaning none of its
descendants intersect the viewport).

…ent.

…es the scroll event listener with an IntersectionObserver.

…erver will handle this case.

…ing construction. (...)

Reading `childNodes` at this time violates the "Requirements for custom element
constructors and reactions".

https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-conformance

Commit ed9b9a3 makes this check unnecessary by making sure that all nodes are
processed by the MutationObserver callback before any update.