[MilanKlausz]

On macOS the G4OSG viewer (--viewer) crashes with Illegal instruction: 4 (SIGILL). It happens after the
window has been resized (F/F5 fullscreen toggle, resizing by hand, or the window being brought to the front),
as soon as osgViewer restarts its draw thread, e.g. when the axes are toggled (CTRL-A) or the hover text
changes.

The crash report always has the same stack in the draw thread:

AppKit                    -[NSOpenGLContext update]
libosgViewer.3.6.5.dylib  osgViewer::GraphicsWindowCocoa::makeCurrentImplementation()
libosg.3.6.5.dylib        osg::GraphicsContext::makeCurrent()
libosg.3.6.5.dylib        osg::GraphicsThread::run()
libOpenThreads.3.3.1.dylib OpenThreads::ThreadPrivateActions::StartThread(void*)

with the message -[NSOpenGLContext update] must be called from the main thread if the context has a view.
After a resize, GraphicsWindowCocoa calls [NSOpenGLContext update] in the next makeCurrent, and AppKit
only allows that on the main thread.

The fix: on macOS (#ifdef __APPLE__) the viewer uses osgViewer's SingleThreaded threading model, so all
rendering is in the main thread. Other platforms are unchanged.

Tested on macOS 26 (Apple silicon), simple-build-dgcode 2.0.1 from conda-forge, OpenSceneGraph 3.6.5,
Geant4 11.4.3:

• unpatched 2.0.1 viewer: the crash above, reproduced again with 2.0.1 (also seen many times with 1.x);
• patched viewer (the same change in a local copy of G4OSG, in use since 2026-10-29): no crash.

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