The dashed indicators for elements outside the canvas stayed pinned at an element's
old position after a move or a seek: they were recomputed by an effect keyed on
compRect/activeCompositionPath, neither of which changes on an in-place soft-reload
edit or a playhead seek.
They now recompute via a MutationObserver on the preview document (coalesced to one
recompute per frame), so they follow the element live. Crop-hugging is preserved.
Adds a regression test that mutates an element in place and asserts the indicator moves.