occQt | A simple OpenCASCADE Qt demo

I haven't used GTK since university, so my experience is pretty basic here.

There are two basic approaches for embedding OpenGL-based viewer into GTK:

1. Ask OCCT to create OpenGL context for a native window taken from a normal Widget or entire window.
2. Wrap existing OpenGL context created by GUI library itself, e.g. Gtk::GLArea.

Your current code tries to follow the first approach used by conventional samples for Qt Widgets and MFC coming with OCCT. I guess it should be feasible, but implies some limitations and issues with mixing GTK widgets, as GTK will not be aware of OpenGL usage.

In contrast, Gtk::GLArea looks like a "modern" way for embedding OpenGL renderer designed by GTK developers and expected to work transparently.

Therefore, I've tried implementing a Hello-World sample using Gtk::GLArea (based on a development snapshot of OCCT 7.6.0dev):
https://github.com/gkv311/occt-samples-gtk

I don't bring the whole code of the sample here, as it is quite large in size. Putting OCCT Viewer into Gtk::GLArea includes some tricky parts like:

• Wrapping native Window into Aspect_Window (it could be also Xw_Window like in your sample, more general Aspect_NeutralWindow or another subclass).
• Wrapping OpenGL context created by Gtk::GLArea into Aspect_RenderingContext/OpenGl_Context.
• Wrapping offscreen buffer (FBO) used by Gtk::GLArea for rendering content into OpenGl_FrameBuffer.
• Putting all viewer redraws into dedicated callback for Gtk::GLArea::signal_render().
• Redirecting user input to viewer (with help of AIS_ViewController).

It is important to note, that GTK may be run in different context:

• X11 server - X Window is created and GLX is used for OpenGL.
  This is default OCCT configuration for Linux;
• Wayland - native window is not X Window and EGL is used for OpenGL context.
  OCCT does support EGL but as a dedicated configuration as alternative to GLX, while GTK handles this in runtime somehow. In addition, OCCT does not (yet) provide any wrapper for a Wayland native window, though it might be not critical for using.
• GTK also has an option to use OpenGL ES instead of OpenGL.

Initially I expected Gtk::GLArea to work natively, but instead a very basic sample (without OCCT viewer) displays artifacts to me (widgets randomly blacked) on Xubuntu 18.04, though it works as expected on Ubuntu 21.04 (within Xorg session). I don't know if it is a bug fixed in GTK implementation, or there is something that should be fixed in a sample to workaround problem on older Linux.