Black screen and vtkOutputWindow errors when replacing interactor in QVTKOpenGLNativeWidget (VTK 9.5.2 + Qt 5.15.0)

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We’re upgrading a Qt/VTK application from VTK 9.0.5 → 9.5.2 (Qt 5.15.0, MSVC 2019 x86, static linking, no CMake).
Our custom interactor — a subclass of vtkGenericRenderWindowInteractor — worked flawlessly in VTK 9.0, but now in 9.5.2 it leads to:

  • black screen,

  • vtkOutputWindow with errors like:

    • "Error: No OpenGL context found",

    • "GenericOpenGLRenderWindow: Window is not mapped",

    • "Unable to initialize OpenGL",

  • crash or infinite loop if Initialize() is called manually before setRenderWindow().

:white_check_mark: With the default interactor (i.e. letting QVTKOpenGLNativeWidget create it), rendering (including NIFTI) works fine.

But we need a custom interactor (e.g. to override CharEvent, add timers, sync external devices), so simply changing the style isn’t enough.

Our current approach (which worked in 9.0):

auto myInteractor = vtkSmartPointer::New(); // vtkGenericRenderWindowInteractor subclass
myInteractor->SetRenderWindow(renderWindow);
myInteractor->SetInteractorStyle(vtkSmartPointer::New());

// In 9.0: widget->setRenderWindow(renderWindow) handled the rest.
// In 9.5.2: OpenGL context never becomes valid → black screen.

widget->setRenderWindow(renderWindow); // ← expected to bind Qt adapter and initialize safely
widget->show();
renderWindow->Render();

We verified:

  • VTK_MODULE_INIT(vtkRenderingOpenGL2) and VTK_MODULE_INIT(vtkRenderingQt) are present in main(),

  • renderWindow is vtkGenericOpenGLRenderWindow,

  • pipeline data (e.g. vtkNIFTIImageReader) is valid and renders with default interactor.

:magnifying_glass_tilted_left: Hypothesis: VTK 9.5+ became stricter about OpenGL context lifecycle and Qt interactor binding — especially around when and how Initialize() is called on a user-provided interactor.

Main question:
What is the correct, minimal, and safe way in VTK 9.5.2 to inject a custom vtkGenericRenderWindowInteractor subclass before QVTKOpenGLNativeWidget::setRenderWindow() — such that:

  1. Qt event binding (QVTKInteractorAdapter) is preserved,

  2. OpenGL context is correctly created and activated,

  3. No manual Initialize() is needed (or it’s done safely).

Is there a new required override (e.g. SetInteractor(), Enable()), or a timing-sensitive step we’re missing?

Thanks in advance — this seems like a subtle but critical change in interactor ownership between 9.0 and 9.5.