So, steps for reproducing:

http://examples.sencha.com/gxt/4.0.2...:basictreegrid

Expand, either with the mouse or the keyboard, until you are here:
Beethoven, Quartets, Six String Quartets.

Then press left 2 times.

You will see that after that Quartets is collapsed, correctly.

But then, no navigation is possible anymore, since the focus is somehow lost.