The copybrush is a quite interesting tool. It allows copying an arbitrary part of a picture to another place or to another picture window.
The tool settings allow choosing, whether the distance between source and destination will stay the same, even if the mouse button is not pressed. Another option is Merged Copy. With Merged Copy the source data is copied "deep". Not only the active source layer is used, but also all visible data of the source document, whether they belong to the active layer, or to an underlying layer.
To use the copybrush, you have to do the following: