@SGaist First thing I tried was mapToScene(), but that didn't work.
What worked, is defining the sceneRect for the scene. Without it, there apparently are no native coordinates and they get set on the fly with each button drop, with the first drop being dead center in the scene.
All I had to do is add the following line:
self.graphics_scene = CustomGraphicsScene(parent=self)
# ADDED THE FOLLOWING LINE:
self.graphics_scene.setSceneRect(0, 0, page_size, int(page_size*math.sqrt(2)))
self.graphics_view = CustomGraphicsView(self)
There's still some polishing like scrollarea showing the scrollbars, basically just not slightly showing the full rect. If I increase the size of the scrollarea, it doesn't solve it. But that stuff is for worries later on.
Thanks anyway, you're a great help!