This looks gooooood...
Dunno if it's been mentioned, but observation (I'm not a UX designer, so apologies for the terms used):

On the last pane to the right, if the purple vertical line delimits where panels extend, and the purple button is used to pull that, it would be great to have the button itself be freely moved by the user up or down.

When I first looked at this the first thing I thought to do was resize the panels as I see fit, followed by pulling the purple button all the way down - This way I always know where it is, when I need it, but it's not in the way either.