I've noticed a following problem in Beta 3 (I'm sure, that it didn't existed in Beta 1 and I haven't installed Beta 2): when I maximize a window (SQL, command or program - doesn't matter) when working in MDI, PL/SQL Developer switches to SDI - a window list bar appears on top of the window and the View -> Single Document checkbox gets checked. When I uncheck it, all windows go back to the normal (non-maximized) state, but when I maximize any of them, the whole problem returns.

This is by design. If you do not want the SDI tab pages, go to Preferences > User Interface > Appearance and select "Don't show tabs" under "Settings for Single Document Interface".

Marco, thanks for explanation.