@Christoph-Hart working well so far but there is one issue:
The draggers are obviously drawn at the top z level.
This means that if you switch your 'main' panel (like FX / ARP / MAIN or whatever you've designed) the draggers remain on top of whatever the new panel is, even if the ui control is hidden, but it's parent panel isn't. The draggers seems tied to their panels - is that correct?
So I guess we have to hide the panels that our ui controls are in, and if, like I've recently done, you've moved all your main UI controls into one 'base' panel, you need to put them (back) into other panels in order to hide the draggers too.
Not the end of the world for me by any means but thought I would flag it up.