I work using my laptop and a separate monitor using "Extend Screen". Up until recently I could move Power BI Desktop to the top screen and it would fill the screen in all the normal ways (drag window to top of screen, click the icon at the top right etc). Now if I try to do that the Power BI window sits in the top left of my screen, and stays about the same size as my laptop screen. Clicking and trying to stretch the window doestn work.
Periodically if fixes itself, but I havent spotted the pattern to that yet. Not sure when this started happening, but it's definitely relatively recent. Never had this before, and all other programs working fine.