GlassMorphAnchor class

The widget a GlassModalSheet morphs out of, and the channel that empties it while the morph is in flight.

Close to an opaque token: GlassMorphTrigger creates one, hands it to its builder, and disposes it; callers pass it straight to GlassModalSheet.show as morphFrom and never touch it otherwise. All it reports back is isPresenting, for chrome that has to know whether the trigger it drew is currently standing in for a sheet. The constructor is private so one cannot be made by hand — an anchor with no trigger behind it has nothing to empty.

Why a token rather than a bare GlobalKey

A key can only locate the trigger. Reading its rect is enough to aim the morph, but not to make the trigger look like it empties — and a trigger that stays painted while a glass droplet inflates on top of it reads as a duplicated button, not a morph. Nothing in Flutter lets one widget hide another it does not own, so the trigger has to cooperate: GlassMorphTrigger owns the key and the opacity, and this token is how the presented sheet reaches back to it.

GlassMenu does the same thing internally with its own trigger; this is that arrangement made available to a trigger the sheet does not own.