Currently, if you make more than one top-level window, at least with the GL-plugin, it will crash with a qWarning. This is not good enough.
We need to investigate our story here on both the raster- and GL-plugins, and document the limitations and make sure that we handle it gracefully rather than crash.
There are certain use cases where this is important, so we should also investigate the possibility of enabling support for it. However, this might not be feasible for 5.1.