When InDesign is maximised, there is still a tiny area right at the top of the screen which doesn't register as part of part of ID's window. If I move my cursor to the very top of my screen, and click in this area, the app or window behind ID is activated and brought to the front. The same thing happens even in presentation (full-screen) view.
This appears to be related to a display scaling issue. I'm using a laptop with a screen set to 150% scaling, connected to a second screen which is set at 100% scaling. The issue only affects the second screen, not the laptop screen.
See the attached screenshot, when ID is first opened. This shows ID on one screen, and part of the laptop screen to the left. The black area above the ID window is due to the difference in the screens' resolutions. Notice the 1px white border on the top and left edges of the ID window. If I restore the ID window, then maximise it again, these white borders disappear, but there is still an invisible gap in this area.
This is a minor but really frustrating issue when I want to quickly click and drag the window's titlebar to move the window to a new location - I end up selecting the window behind.
This is ID version 16, but I don't remember when the issue started (it has been a long time). Photoshop and Illustrator aren't affected in the same way, or any other apps for that matter. Running on Windows 10.
Possibly somewhat related, I still sporadically have issues with ID not opening maximised, even when it's been set to, or when it was maximised prior to being closed. The window will open nearly-maximised, i.e. a regular window, filling the screen with a ~20px gap on all four sides. I know this has come up several times over the years, but I thought it had been fixed. I still can't identify the pattern for when this happens or not.
@interiorsb49021866 Yes, many people have noticed the problems with the UI scaling in InDesign. InDesign has UI scaling controls in Edit / Preferences / UI Scaling so check your settings there to see if you get any better results. You have to restart the program to see the changes. InDesign will not "stick" to maximize to full screen on launch for me, either. Probably the best thing you can do is to vote for this to be fixed in an upcoming release. InDesign forgets the maximized state of the window – Adobe InDesign (uservoice.com)