This issue has been posted before, with no solution. Ever since one of the more recent updates to InDesign, the application window no longer automatically scales to fit the screen when connected to an external monitor and, when it's connected, the application window is blank—showing only the pasteboard color across the whole application window.
So you have to hit the "+" to expand the app window to the full screen area, close the file window, and reopen the file in order to work again.
Similarly, if you hit "Shift + W" to view Presentation Mode, the application window always re-acquires the window size from the laptop. So you have to hit "+" to expand it every time.
These are bugs that never used to be a problem ... and they take tons of micro-minutes away from my standard workflow.
These are really important issues to resolve. I'm surprised they have continued for so long ...