After updating SourceTree to 1.9, I did something that causes my 'Working Copy' view to always display only a single file in diff mode. I can find no way to get it back to showing me my list of files. If I select to display 'pending', I see one pending file in diff mode. If I select all, I see one (different) file from among all the files.
I've tried refresh, reinstalling, and cloning a new project from bitbucket -- no change.
I even tried reverting to 1.8.1 -- same behavior with older diff view, then upgrading to 1.9 again. Nothing helps.
Looks like you hit a bug which causes your view to be remembered like that. This has been fixed now, but if your preferences are set like that then it'll stick around regardless of what version you install. To fix this, on the left of that pane there's a border which is twice as thick as other borders. This is because it's two borders stuck together (both are splitters that can be adjusted). You can reveal that view again by grabbing the right hand one, which is quite difficult as they're 1px wide, but it is do-able. Grab the right hand splitter line and drag it to the right to reveal the missing view.
Hopefully that makes sense, but if not, feel free to question the problem again.
I just ran into this issue with 2.0.3 -- thanks so much for the answer, this worked perfectly. A suggestion for the Atlassian folk is make it so that this middle-windows cannot be collapsed completely -- I can't imagine any use-case where you'd want to completely hide it anyways.