I'm on Linux Mint 17.2 using a FullHD laptop and a FullHD external screen. Ocasionally, some apps, namely Firefox, will open dropdowns in the wrong screen, for instance, trying to open the menu in Firefox in the external screen shows the menu on the laptop screen.
I couldn't find anything specific to this issue and I can't point out at this point whose fault is it: Firefox or the OS. I'm not sure if this happened in other apps so far. The issue only seems to arrise when the window is in the external screen (i.e. in the non primary screen).