onBack property
What the Android/Flutter back gesture does. Separate from onClose
because the two are different intents: × is an explicit "I am done with
this", back is "undo the last step". Defaults to collapse for that
reason, and authors who want back to end the campaign can say so.
iOS has no back gesture, so this is inert there — which is why the collapse button exists.
Implementation
final PipExpandedClose onBack;