clear

Excludes nothing for now, while STAYING registered — for content that is momentarily off-layout (collapsed to zero size, hidden) but still on-window, so a later update revives it. Distinct from unregister, which removes the entry entirely (used only when the content leaves the window). Not the same as update-ing an empty rect: a zero-size AxRect still sits at a point and would deafen a tap there, whereas a null rect matches nothing. Idempotent.