resized property
The demo was given a different box — so every rect in the tree is out of date, though not one widget in it changed.
Its own signal rather than folded into structureChanged, because the shape did not change and saying it did would be a lie the host could act on. It is also the commonest real staleness there is: dragging the panel divider resizes the preview, and until this existed the tree went on reporting the widths it had before the drag — including whether anything overflowed.
Implementation
final bool resized;