Find the resolved rect of the (first) node tagged id (see Widget::id).
Meant for baking a widget tree’s computed positions into some other
system at “measure time” (e.g. a mod reporting real-slot pixel offsets to
its host) — a WinForms-style measure-then-use pass, not a per-frame call.
Hit-test: find deepest clickable, enabled node at (mx, my).
Children only count where they’re inside their parent’s rect, so rows
scrolled out of a clipping container can’t be clicked.