No user visible change expected (this isn't used in existing code yet).
Users of the tree-view API should be able to write their own `matches()`
method to compare tree-view elements after tree reconstruction. Part of
the tree-view matching process wouldn't use this though, and use the
`matches_single()` method instead, causing issues like multiple items
sharing the same state. This was initially done for an optimization
(the default `matches()` compares parents as well, which seems redundant
here), but it backfires and actually isn't needed. The default
`matches()` only compares the parents when `matches_single()` returns
true anyway, so the redundancy is really minor and not a performance
concern.
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