Creating a new tab when full-screen visor is opened shinks size of window

To reproduce, show a full-screen visor window and hit apple-t to create a new tab. A space is usually created on the right, and/or bottom, and doesn't go away when you close the tab. This only happens with the first tab.

Start by reproducing the issue with a full-screen visor window, then inspect the code handling the first tab, new-tab creation, and visor window sizing. Done means creating and closing a new tab no longer leaves extra space on the right or bottom of the window.

Written by the indexing model from the issue text.