Some applications still react funny when they are moved from one display to another, especially if the other display has a different resolution - poorly written applications don’t check to see if the last position is actually visible on the new display (E.g. you went from a 4K monitor to FullHD and the window was bottom right on the big display).
If an application windows doesn’t show up, even though the application has started, it is possible that the position is “off screen”, positive or negative. You can’t pull the window back on screen with the mouse, because it has no visible title bar to drag.
To get around this, use the following trick:
- click once on the icon in the task bar
- Press Alt + Space (opens the window’s control menu
- Press cursor down once and press Enter (alternatively press “M” for move - in German it is V for Verschieben, I think it should be “M” in English, but double check on a visible window)
- (If you move the mouse, nothing happens, don’t worry)
- Press an arrow key, it doesn’t matter which. The mouse pointer jumps to the part of the screen where the window is hidden (left, right, top, bottom).
- Move the mouse until the window appears on the screen
- Place the window somewhere visible and click the left mouse button.
Et voilà, the window is visible and usable again.
Edit: You can test this by dragging a window down behind the task bar and leaving it there, then going through the process above.