Others, refuse to be gathered. A canonical method to force all windows which exist off-screen to be shown at least partially on SOME screen would help
And something like a neon flash border (like zoom does on screencast panes)
1) Under the Window menu, select the window that's gone invisible 2) Minimize it (using the Cmd-M shortcut or the Window menu item), causing it to become visible in the Dock 3) Click on it in the Dock, un-minimizing it and making it visible again
Now, I can't guarantee this would work generally (I was only having the problem with a particular Safari window that I regularly kept in split-screen with Music on my external monitor), but it did work consistently for my particular issue.