I think it is quite standard behaviour nowadays and is IMO intuitive enough. I guess it is feature of the GUI framework used in PSPad, your question made me to mess with sidebars and its items for a while and I was surprised what can be done with them: move code explorer into the left sidebar, detach something from left and attach it next aside or below right sidebar item. Not many changes are saved across sessions though. I would not be surprised if not all of these features weren't fully planned by mr. Fiala :] You are right that small note in documentation would not spoil anything.
-- <http://forum.pspad.com/read.php?2,57291,57295> PSPad freeware editor http://www.pspad.com
