Patching Rectangle size doesn't correlate to Window Size?

I set the patching rectangle of a Panel to 800 x 600. Then I set the window size to 800x600. I would think this would make the Panel fill the window perfectly. But in fact the window is much smaller and the panel runs over both edges, with a horizontal and a vertical scrollbar.

I'm not clear on why this is happening. I would love to create some attractive fixed sized windows.
Thank you!
See attached: