[vtkusers] Custom VTK Cocoa window flickers gray on new macOS

[Alessandro Volz]

Hi,

I am a contributor to the Horos project (the FOSS inherited from OsiriX). In this project, we use VTK for the Volume Rendering window. When I say 'custom', I mean that we assign our own NSView and NSWindow to the vtkCocoaRenderWindow (through SetWindowId and SetRootWindow), and initialize the vtkRenderWindowInteractor inside the first drawRect: call.

With the latest macOS update we have this strange behavior in the VR window. Somehow, the VTK render sometimes doesn't successfully get drawn on screen:
- the window is initially gray
- when resizing, it flickers between gray and render
- when calling the underlying interactor, the resulting updated image is rendered on screen (for example when rotating).

Do you have any advice how to fix this?

Thanks,

Alessandro