Bob,
I feel this setting should be in the "Options > Video" area, not "Tree & View". I couldn't find it for awhile because I was looking in the wrong place.
Here is what I expected: MC 23.0.14 set up to open and run on monitor 2 (NEC 24x12) at all times. From monitor 2 you select "Video" and click on a movie to play. The movie starts playing on monitor 1 (main monitor - Acer Predator 34x14) AND MC 23 continues to be visible on monitor 2 for control purposes. Please consider adding a setting to force MC23 to open on monitor 1 or 2. It seems to insist on opening on main monitor 1 all the time and I have to drag it over to monitor 2 and resize the window.
Here is what happens to me twice (Ubuntu 17.04 Gnome Desktop):
With MC 23.0.14 running on monitor 2, selecting a movie causes the MC23 program on monitor 2 to disappear (the only thing visible on monitor 2 is the desktop background) and the movie begins to play on monitor 1.
If you "stop" the movie, MC 23 reappears on monitor 1 when it had been on monitor 2, albeit, invisible. (I don't think that's supposed to happen). Also, if I click on the "X" which appears in the upper right when you move the mouse to the top edge of the monitor 1 screen, the movie playing on monitor 1 jumps back to monitor 2 and continues playing. If you then stop and start the movie, expecting it to go back to monitor 1, it doesn't, it continues to play on monitor 2. I also experienced stopping the movie on monitor 1 and simply having MC23 reappear on monitor 1 with no evidence that the movie was playing.
If I exit and restart MC, it seems to reset itself and initially plays the movie on monitor 1 like before.
A few hours since the above tests, I rebooted to Ubuntu 17.04 Unity desktop. It's worse. I can't get the movie to play on monitor 2 at all. It will only play on monitor 1 even though I have set monitor 2 as the default for movies and run MC23 from monitor 1.
By the way, I previously reported "tearing" issues during movie playback which you couldn't duplicate. It seems they only exist on monitor 1 (Acer Predator 3440x1440 using DP input on Nvidia GTX1080) and NOT on monitor 2 (NEC 2490Wuxi 24x12 using HDMI input off same GTX1080).