Can't have TextEdit and Indesign open on screen at same time

For years happily had both open on screen to copy text from one to the other. Suddenly I can only have one or the other open. If I have Textedit on screen and open an Indesign file, the Textedit app disappears from screen (or vice-versa). Then I hit the Indesign icon on the dock, the Indesign file comes back on screen but Textedit disappears (again, and vice-versa). What can I do?

With thanks,

Larry