[Reg 6.2->6.5+] Qt::AA_ShareOpenGLContexts has stopped working for offscreen rendering

Customer requirements

• The app grabs the contents of a QGraphicsView with a QOpenGLWidget viewport, containing standard QGraphicsItems as well as items that paint using direct OpenGL calls.
• The grab must work regardless of whether the QGraphicsView is visible on screen or not, even if it was never shown yet.

Problems

• The basic grabbing methods, using QGraphicsView::grab() or QOpenGLWidget::grabFramebuffer(), don't work – they are missing the OpenGL content.
• A workaround to the above was found: Add an additional "offscreen" QOpenGLWidget that renders the QGraphicsView in its paintGL method, and grab its framebuffer. This worked fine in Qt 5.15 and 6.2.
• In newer versions of Qt, this workaround no longer works.

To reproduce, build and run the attached project in Release mode, using Qt 6.2.11 first, and then using a newer version of Qt.
 

Expected outcomes (Qt 6.2.11)
Most tests will produce a green square and a coloured triangle on a white background (but the "viewport" method only works if the QGraphicsView was shown first)

 
Actual outcomes (Qt 6.5.5/6.6.2/6.7.0beta3)
The "viewport" method remains the same as before, but the "offscreen" method no longer captures the coloured triangle at all.