Ah, that's not supposed to be there like that indeed. I'll probably need to snoop around Godot forums to figure out how to test/avoid this on other machines. In the project and my system the entire screen space is utilized. Thanks for spotting it!
Probably has to do with the stretching. Try changing the values in Display->Window->Stretch You can replicate this by resizing your windows, or zooming your browser page to other than 100%