When using multiple monitors on macOS, panels, windows, and other interface elements that you've placed on a secondary monitor move back to the main monitor after switching to other applications and back.

Solutions

Solution 1: Deselect Displays have separate Spaces in Mission Control.

1. Select the Mission Control icon in the Dock, or choose the Apple menu > System Preferences > Mission Control.
2. Deselect the option Displays have separate Spaces.
3. Log out and log back in.

Solution 2: Assign the application, such as Photoshop, to All Desktops or None.

1. Command-click or right-click the application icon in the Dock.
2. Go to Options.
3. In the Assign To section, select either All Desktops or None.

Note: If you do not see the Assign To section in Options, it's possible that you disabled additional spaces. Create a temporary space so that you can see the Assign To options. For more information on creating spaces, see Work in multiple spaces on Mac.

Additional information

MacOS gives you the option to assign an application to a specific Space. Doing this forces an application to always opens in this Space, which helps with organization and reducing screen clutter. However, all elements of that application must also exist in that same Space. The OS allows you to temporarily move elements to another screen while the application is active, but when switching to another Space or application and back, the OS assigns all elements back to the designated Space.

Other issues that are solved when this option is deselected are:

• You are able stretch application windows across multiple monitors.
• You can drag panels in Photoshop to your secondary monitor.
• Lights Out mode in Lightroom affects all monitors.