It's near-impossible to dock a plugin on OS X. I tested on OS X 10.11.6 (El Capitan) on a Mac MIni and a mbp. It's working flawlessly on Windows and Linux, but it's a horror on OS X.
Docking left, top, right and down of the stdmultiwidget is "ok" when you're in the right zone. That is, in a really specific zone near the middle of the render windows. It's not particularly great but it works.
If you're anywhere else though, the shadow of the plugin flickers when you move. As if Qt understood only for 1 mouseevent that the position was right, but then it's lost and the plugin detaches if you release the mouse.
I did succeed one time, moving the mouse really fast and releasing randomly, hoping to be in the right millisecond :) It was impossible to put a plugin on another plugin though (creating tabs).