Qt Quick: Support for lightweight popups, tooltips and other sub-windows exceeding frame of parent window

Qt Quick lacks a feature to display tooltips, context menus, combo-box dropdowns, popups and similar sub windows outside of or exceeding the application window boundaries.

Firefox, for example, features a long context menu which is rendered as a separate Window (see attached screenshot). 

To create similar behavior in Qt Quick, we would currently need to resort to a QQuickWidget which is positioned absolutely on the screen. However, a noticeable delay with a brief white flickering occurs from creating the widget until the popup being really displayed, probably due to a separate QML context being created for the widget and not sharing resources with the QML backend from the main window. The new feature should support pre-rendering of the popups to be ready to be displayed quickly without lags, just like when changing the "visible" property to "true" on a regular Qt Quick element.

A great bonus would be transparent background for the popups.