Dual monitor applications opening on wrong monitor
Solution 1:
If you do not have CompizConfig, install it (you need it anyway).
sudo apt install -y compizconfig-settings-manager
Go to CompizConfig Settings Manager > Window Management > Place Windows (make sure that this plugin is checked). Under the General tab you see the option Multi Output Mode. You have options like: Use Active Output Device, or Use Output Device with Pointer etc.
Solution 2:
Something similar to this happened to me a while back. Try going to System > Preferences > Monitors. Make sure that the main monitor (your working monitor) is to the left and not the right. If the two monitors are different resoutions make sure that they both align at the top. I noticed this with my setup a couple months ago when I installed a monitor that was using a resolution way higher than my working monitor.
Solution 3:
Another choice is to use devilspie. You can get Devilspie from the Universe repository.
Solution 4:
I believe (if you have compiz enabled) that you can do this from the compiz settings manager using the place windows plugin. I've not done this myself, but if you play around a bit with the options you should be able to get the window to appear on the desired monitor.
Solution 5:
In System->Preferences select Monitors. Check "Show Monitors in Panel" then click on the monitor you want as default and hit the "Make Default" button. Log out and back in and there you go.
I've tried this a few times and it never took until I checked the "Show Monitors in Panel" box.