The mxn multi widget can be used to display different images in parallel. The user can easily compare different modalities and navigate individually in each render window.
In an additional use case the user wants to enlarge one of the render windows to have a better view on the image while working on it. The StdMultiWidget allowed this via the render window menu. The render window menu is currently not available (see T24215) so we need to provide a simple way to enlarge the currently active render window.