This occurs if you re-size your browser window , while you have a landmax window open. 

There are a number of options you can do:

- Double click the header of the window, and it will match the screen size. 

- Make your browser full screen and then close the window

- On the task bar on desktop - you will see a tab for the window - right click and options to close the screen will appear

The window resizing for Chrome on Windows, should work if you do a quick double click on the Tab. 

See attached screenshot.