In MITK 2016.11 the intention of QmitkRenderWindow was: handle Qt mouse events inside MITK and only if no MITK interactor is interested, then show the Qt event to QVTKOpenGLWidget, let it do further handling such as telling VTK interactors about it.
This intention seems still valid and helpful. It allows, for example, to implement a MITK interactor that would change a paint brush radius via "modifier+mouse wheel". MITK would do that and VTK would neven attempt to do zooming because of the mouse wheel.
In MITK 2018.04 (and, looking at the sources, current master) QVTKOpenGLWidget changed considerably and QmitkRenderWindow did not. Consequently, the Qt events are now always _first_ processced by VTK's interactors (in the example: execute zooming) and only afterwards handled by QmitkRenderWindow and MITK interactors. This breaks interactions as the paint brush example above.
I am surprised that nobody ran into this yet (no interactors that use the mouse wheel anywhere in DKFZ?). Anyway I need this fixed in my application and will attempt to work on a fix soon. I would appreciate if somebody from the DKFZ team could support with review and testing (I guess there are quite some interactions implemented on your side and I lack overview)