OCC & VTK viewers on top of everything

Hi,

I'm working with Salome-Meca V2018.01. on virtualbox.

When I'm in the geometry module I'm facing a bug with the OCC viewer. It stays  always on top of any dialog window. In the attached file I show how it's over the point creation dialog window. It also happens with system dialog windows such the one appearing before shutting down the computer. 

It also happens with the VTK viewer, but if I close both of them everything is fine.

I have 3d acceleration activated in virtualbox, if I turn it off, salome.meca simply doesn't start at all.

Has anyone any idea of how can I overcome this issue with the 3D viewers?

thx in advance