my hardware setup in the office ist the MacBook Pro as the 2nd screen and an Eizo over Thunderbolt as my main screen.
When I work in Lightroom, the main window is on the Eizo and I need the 2nd Screen on the MacBook Display. For the first time it works, like the time before Lightroom Classic CC but when I change to another app and get back to Lightroom, the 2nd window appears in front of the main window on the Eizo. It is not possible to bring the 2nd window back to the 2nd display... no way! Sometimes for a few seconds, but then it appears back on the Eizo again.
The system was clean installed with the change to High Sierra and the first Version of Lightroom on the fresh system was the Classic CC.
The same problem I have at home with an external display over hdmi. But there I want to have the MacBook as the main display and the 2nd screen on the external display and the problem ist the same.
It suggests that the 2nd screen in Lightroom only works on the display thats set as the main Display (main desktop where also appear the folders and drives and datas).
Do anyone has an answer?
I have the same two screen setup on my iMac (where the iMac is the second screen) and I have no problems at all with it. Check the following: in MacOS X System Settings you can check/uncheck that each screen is a different workspace (with its own menu bar too). I have turned this off, so only my main screen has a menu bar. The reason is that I had found in the past that both Lightroom and Photoshop have problems if you turn this on. As I'm typing this on my iPad I can't post a screenshot with the exact settings, sorry.