Problem: You used to be able to drag a maximized window to "un-maximize" it. Now you can't.
Solution: Per this post at SuperUser, uncheck "Prevent windows from being automatically arranged when moved to the edge of the the screen" in Control Panel / Ease of Access Center / Make it easier to focus on tasks.
Note: It might take several seconds for Windows to apply this setting after hitting "OK" or "Apply". If you test it too early, it may seem like nothing changed. Wait 10 seconds and try again before deciding that unchecking the feature did not work.