For some reason I jumped to expecting this to morph the display in the direction of slouch, (such as tilting and shrinking into the lower-left corner a bit say) encouraging you to move opposite (sit up/lean right) to 'get a better view' as though the monitor were a window into a 3D world.

Probably hard to implement, but have you considered anything like that? I imagine it'd be quite a nice subtle hint, that you'd subconsciously correct for even.

Just seeing/reading this submission made me correct my posture though, thanks :/.