Where a tree layout hangs from, as the Physics flyout's Root card offers it: a node the tree is pinned to, or — with nothing pinned — the finder that picks one.
The two are exclusive: pinning a node retires the finder until the pin is dropped.
The finder that chooses the root while nothing is pinned.
Optional
The node the tree is pinned to; undefined while the finder decides.
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Where a tree layout hangs from, as the Physics flyout's Root card offers it: a node the tree is pinned to, or — with nothing pinned — the finder that picks one.
The two are exclusive: pinning a node retires the finder until the pin is dropped.