Currently, when only one tab is open, you can't close it. This is not intuitive at all, and since the close button isn't disabled, it's probably a violation of both Windows and Mac UX guidelines (but I'm opening the bug here because I primarily use SourceTree on Windows).
What I'd like to see: if one tab is open, you allow it to close, and you close that tab and show the list of bookmarks in its place.