anchor property
What the tooltip is anchored to: the child's rect, or the pointer.
The tooltip attaches to the node's icon-and-label content, not to the whole rendered row. But a row is as wide as the tree's longest label, and each label grows to fill its row — so a long label's rect spans the row, and a tooltip anchored to that rect appears at the row's centre, far from where the user is actually pointing. Worse, once the tree is wide enough to scroll horizontally that centre leaves the view, and the tooltip is drawn outside it (see issue #47).
TooltipAnchor.pointer keeps the same hover region but places the tooltip at the cursor, which is inside the view by construction.
The anchor is captured when the tooltip is shown and does not follow the pointer, so interactive tooltips stay reachable. Tap-triggered tooltips anchor at the tap; a controller-driven show with no pointer present falls back to the child's rect. Against a point there are no target edges to align to, so alignment then selects which of the tooltip's own edges lands on the pointer.
Implementation
final TooltipAnchor anchor;