Drops the gesture with nothing written.
Commits what was previewed.
Draws the movable at a placement (absolute px) until the gesture ends.
Starts a gesture on the control's movable: the layer snapshots it and readies one commit.
onCommit runs on the movable inside that commit, after the previewed placement is written.
Returns false when the layer refuses gestures.
OptionalonCommit: (movable: IMovable) => void
What the layer lets a content's own control do: run a gesture of its own through the layer's preview and commit loop, so it is drawn, cancelled and undone as a move or resize is.