Here are 3 images. I simply colored the QSplitter in red, the widgetContainer in blue and the parent (QmitkStdMultiWidgetEditor) in green.
For example, m_MainSplit->setStyleSheet("background-color:red;");
We can see here that everything is as it should. We only see the red of the QSplitters.
I clicked on Fullsreen axial. We suddenly see the widgetContainer (blue) and the QSplitters (red). Yes, you can move the QSplitters but it does nothing. Also, there's now a little less space for the 4views, you can see it if you open both images.
I clicked on the 4views button. Now this is strange. Some QSplitters are now empty and glued on the top. I'm not sure if there's more and more if we repeat the process. Yes, you can move them but it does nothing. Also, eveyrhing turns green (so, children of QmitkStdMultiWidgetEditor?)
I made a new PR. It's almost identical to the first but at least it's tested on the current master. It still compiles and it still work as it should.
The only problem I see is that it's hard to click on the toolbox (the right-most icon) when you just clicked on it and the views moved. However, it's already a problem on master so I didn't break anything. I don't see this problem on Linux.