A quick click of the mouse in HelloMedia causes the video to freeze (audio keeps playing); if you resize the window is starts working again. I'm pretty sure that any animation will also cause it to start working again.

This is triggered by the same one-shot animation cause by a mouse click that triggered RT-15407. There is likely some code that was relying on a pulse to repaint the scene, when instead a scene with an active mediaview should be repainted on each timer pulse.