I have a similar setup, the arrange view on the primary monitor and the mixer/docker on the secondary. And yes, the mixer flickers briefly when I undo. I haven't noticed this earlier, but now I guess I can never un-notice it! Thanks
This is W7x64, Reaper x64 portable install, ATI Radeon HD 4290 integrated on the motherboard (Asus M4A89GTD Pro/USB3).