The OverlayContainer is quite similar to the ThingInspector control. It is a popup which is positioned over the application content. The OverlayContainer is either aligned with the Shell control if it is available (on the same window) or with the window itself (like in the example below because the Demokit content runs within an IFrame). The difference to the ThingInspector control is that the OverlayContainer allows arbitrary content.
Here is a small example using the OverlayContainer control.