'u' key crashes the plugin
When issuing:
:DirDiff /tmp/toto/ /tmp/titi/
I get this:

First off, the colorscheme is all bugged. I have to enter the command :! to refresh the screen. But the plugin is usable.
Then, when pressing u right after, the two diff'ed windows are closed and the DirrDiff window takes the totality of Vim's region of the screen.

Now the plugin is not usable anymore. Trying to open files just doesn't open no window. I have to quit the plugin twice, i.e. I press q then answer the prompt for quitting, then pressing q triggers the prompt again, then the plugin quits. The screen is still bugged. I have to issue:
:%bd
then ctrl-w followed by shift-_ in order to take back a usable portion of the screen for Vim. Otherwise buffers are limited to a small region of the screen above.
I did try to disable all of my plugins and colorscheme configs and the issue was still there...
I did find a workaround: instead of pressing u, simply call the :DirDiff command again and there are no issues except for the colorscheme drawing which are also worked around with :!.