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Portals and `<ViewTransition>` are tricky because they leave the React tree. You might think of a Portal's container conceptually as also being part of a React tree but that's not quite how they're modeled today. They're more like their own roots. So instead, of trying to find a conceptual place in the React tree we treat Portals as their own root. We have two ways of tracking whether an update to a ViewTransition boundary has occurred. Either a DOM mutation has happened within it, or a resize of a child has caused it to potentially relayout its parent. Normally that just follows the tree structure of React, but not when it's a Portal. When it's a Portal we don't know which DOM parent it might have affected. For all we know it's at the root (and in fact, in most cases that's where Portals go). With this PR we mark the root as having been affected by a mutation or resize. This means that the whole document will animate and we can't optimize away from it. This ensures that a mutation to the root of a Portal doesn't go unanimated with other things are animating such as its parent. You can regain this optimization by adding a `<ViewTransition>` boundary directly inside the Portal itself so it owns its own animation. If that DOM node is also absolutely positioned it doesn't leak. Conversely this also means that a mutation inside a Portal doesn't affect its React parent so it won't trigger its parent's animation if this was the only thing animating. That could be unfortunate if this container is actually inside the same React parent. However, because this would have been an update we would've marked it for "maybe animating" and updates can't only get their animations cancelled if the root is cancelled, in practice this will actually animate anyway.