Whenever I use DPO in fullscreen mode and open up the sidebar drawer, everything is fine. But when I close it, the space the drawer left, remains. The window does not scale to full width again. Bug?
What version of the app and OS are you using? That bug was fixed (for me) in a much earlier release of DEVONthink.
I have the latest versions of DEVONthink Pro Office (2.4.3) ans OS X (10.8.2)