Base class for contextual windows and panels with fixed position.
[Popup] is a base class for contextual windows and panels with fixed position. It's a modal by default (see [member Window.popup_window]) and provides methods for implementing custom popup behavior. [b]Note:[/b] [Popup] is invisible by default. To make it visible, call one of the [code]popup_*[/code] methods from [Window] on the node, such as [method Window.popup_centered_clamped].
Emitted when the popup is hidden.