Quoting myself and you here, but I've been doing some testing
I'm investigating weirdness with multiple screens.
I don't think that weirdness is just confined to a mac.
Just plugged in a secondary monitor that I found in the loft.
Initial findings are:
1. You can see I have 'Backdrop' turned on, but there's no background colour. 2. That's because it's showing only on screen 2. 3. So I created a new stack, which opened on screen 2 for some reason - even though the revMenubar stack and the tools palette is on screen 1. I then did the shortcut for the message box, and it popped onto screen 2. 4. Dragged the message box back over to screen 1, and tried to find out the supposed topleft of the new stack. It reports 289x210 which is obviously wrong. The 'background' is now randomly appearing on Screen 1 and no longer on Screen 2. 5. Back over to Screen 2 and clicking on the stack moved my message box and tools palette over from screen 1 to screen 2 without asking. It also moved the "Backdrop" from screen 1 to screen 2 again. 6. Tried to ctrl-s and save the stack, and the save dialog popped open on screen 1, not on screen 2 where the stack is, but my message box and tools jumped back to screen 1. After saving, my tool palette disappeared 'behind' the background stack, and the only way to get it back was to turn off the background again.
It all seems very random and very broken.