The idea to support "subviews", i.e. a sort of limited Window that can be nested inside of Window, has floated around a few times, so I wanted to open an issue to discuss it.

A Subview would have the following platform equivalent (feel free to edit this):

• iOS: UIView
• macOS: NSView
• Wayland: wl_subsurface
• Web: HTMLCanvasElement (?)
• X11: XID
• Windows: HWND
• Android: android.view.Surface / ANativeWindow

Related: #696. Also discussion in #3311.