Taking an example from ScrollArea, on show functions the panel could return something more like:

pub struct PanelOutput<R> { pub inner: R, pub response: Response, pub separator: Response, }

This would allow developers to have better control over side, top, and bottom panels.

Personally I'd love it for the following use cases:

• double middle click interaction on separator to restore panel to some default size.
• track panel size as a ratio of available size based on drag_released on the separator Response. This way the ratio isn't needed to be updated every frame but only on key event frames.
• create new panels on interaction with the separator instead of requiring other buttons to be available.