This page is the raw control. It reads the DOM selection directly via window.getSelection()
and never repairs it. Use the scenario buttons to switch between several DOM replacement shapes that
all destroy or rebuild the node where the caret used to live.
The goal is simple: find a mutation shape that makes the Selection API go stale again. When that happens, the poll log turns STALE and the red marker falls back to the editor's top-left because the reported range has no geometry.