Hi.
I managed to make callout labels that snap to screen edges like this:
Playground implementation:
https://playground.babylonjs.com/#X9DCCM#13
The idea is to make a ray from center of screen to projected position of the linked mesh and find intersection with a line of an edge (adjusted for margin to fit whole block)
For the GUI control the calculation is performed in _computeAlignment which seems the best place to do that.
