Two monitor configuration: launching a particular app automatically creates 4th screen, toggling required (help!)

Thank you for your note. I apologize that I am not describing the problem well. Everything has been working fine until I must have done some silly unintentional clicks. Famous last words!

I have two monitors: the actual iMac screen (on the left) and an added Samsung monitor (right).

NORMAL: I turn on computer; iMac screen has desktop, dock, bar, desktop files and folders, etc. Everything you would expect. On the Samsung monitor there is only the El Capitan image.

NORMAL: for example, I launch Safari from the dock and it opens up the iMac screen (where I had it open last); and for example I launch Excel from the dock and it opens up on the Samsung monitor (where I had it open last). IMPORTANT: I launch an called Bookhound from the dock and it opens on the Samsung monitor (where I had it open last). This is normal working procedure -- all very boring which is the way I like it.

NOW THE PROBLEM IS: I open Bookhound from the dock and the following happens: the desktop on the iMac screen and the El Capitan screen both shift LEFT, and iMac screen turns black, and the Samsung screen has the Bookhound app (rendered full size).

I can CTRL right one click and the iMac desktop and Safari screen are both where they were upon turning on the computer. I can CTRL left click to get back to the Bookhound app.

QUESTION; what do I do to make it so that Bookhound doesn't create an entirely NEW screen (separate from the two existing screens)?

Once again, I apologize if I am not explaining the problem correctly. Thank you for your patience!

Sincerely,

ML

Doh! Fixed it (the problem involved the ENLARGE (green) button the application in question -- MY BAD

: \

ML