Use case

A an animated "container" widget that can be expanded/collapsed similar to many native iOS apps.
Examples:

• iPadOS sidebar (Add a Sidebar widget to Cupertino for a native look on iPadOS #60762) (source: iOS 17 Music app)

• "Show-on-load" list groups

• Calendar/Time inline widget (Add new Cupertino calendar date picker mode #63693) (source: iOS 17 Calendar app)

• Collapsible groups (source: iOS 17 Mail app)

Proposal

I've already started working on it as a part of #60762 and I'm ready to open a PR to contribute this feature.

I suggest the name "collapsible" due to it being closer to native iOS SwiftUI nomenclature (https://developer.apple.com/documentation/swiftui/section/collapsible(_:))

The suggested API is very simple something along the lines of:

CupertinoCollapsibleContainer(
  collapsed: true,
  child: Text("Hello World!"),
),