After upgrading my Mid-2011 iMac to macOS Sierra from Yosemite (I skipped El Capitan), I have a couple of Mac apps that won't remember their window positions on my external monitor. So far, I've found that iTunes and Mactracker are two that don't. They always open on the iMac's screen.

Also, even after moving the iTunes window to my external monitor, all of its warning dialogs still appear on the iMac..

Oddly, the Messages app opens fine on my external, as does Activity Monitor and one of the two windows I have set to open there with Firefox.

Any idea where preferences for app window positions are contained or what I have to reset for the windows to open where I last closed them?