Found a work round.
If you right click the tab you want to move and open in new window, it opens in a new window. OK.
You can now drag this to the other window you intended to move it to ie your other screen and close the redundant window.
I think this is a bug in CS.
I get many crashes ie the "please wait a moment" box when recompiling intlibs. almost every time.
I've used Altium for 10 years in previous job and there were always little niggles. "please wait a moment" was very common. There was one where if you created a poly on an internal plane ( I know you shouldn't but I forgot it was a plane and not a layer) it corrupted the pCb design and you had to carefully navigate the mouse to save the pcb as ascii and re-open. If the mouse pointer went onto the design it crashed
Sorry: This only worked once! I can't reproduce it.
So I started CS yesterday and made a new project with Schematics and PCB. Then I tried to drag and drop the tabs to the order that I wanted. And that feature is gone somehow. It shows that I am dragging the tab but then it doesn't move when I let go of the mouse button. Is anyone else experiencing a similar glitch?