Sharing a scene with custom material between multiple windows doesn't work

Using same scene in multiple View3Ds in multiple windows doesn't work if there's a custom material in the scene. To reproduce, run the attached example.

It looks like the problem is that QQuick3DCustomMaterial::updateSpatialNode() determines the context when it's first run and the shaders are cached only in that context.

This issue blocks showing custom materials in Design Studio 3D edit view.

QQuick3DEffect::updateSpatialNode() also determines context in similar manner, so it might also have similar issues.