Juard,
This is an excellent question haha!
I hated it so much when this happened. I had to close the window, open another window, load configuration etc…. it was a nightmare just to avoid getting annihilated on a trade.
I actually figured out this answer by accident, and I was so happy when I did!
If you press Alt + Space a context menu will appear that will allow you to Restore the window to its regular size.
Good luck trading,
Bailey Nevener