https://superuser.com/a/1913268 (via) read
This is in the top search results for "windows blue outline around window," and there is a related "issue" if you have PowerToys installed, which has a feature called "Always on Top" that keeps the window above all other windows.
In that case, select the window with a colored border, and you can enable/disable always on top by hitting
ctrl+win+T
This added valuable info not yet found in the other answers. A user made a baseless claim discrediting it without evidence. It was downvoted and deleted. Please undelete so I can add a link to PowerToys Always On and add a screenshot showing this behaviour.
