How to reproduce: 
1. In Preferences/Theme choose 'Default'.
2. Switch to multi-window mode.
3. If necessary remove all dockable dialogs from below or next to the toolbox to have the toolbox in its own single floating window.
4. Drag one dockable dialog and attach it on the right side of the toolbox.

Expected result: 
The floating toolbox window is resized to show both the toolbox and the newly docked dialog.

Actual result: 
The floating toolbox window is not resized, see attachment. 
The users wonder where the dockable dialog has gone. If they realizes that the dockable dialog is in the expected place, but invisible, they have to fumble around with the conflicting vertical split divider and the right window border to resize it and make the dialog visible. Usually they grab the split divider, but only the window border resizes the toolbox window. For Mac users there's no right window border to resize and the window resizing handle in the lower-right corner can be invisible, too (see bug #686860). So they to switch to single window mode and back with all the side-effects and hassle of losing window positions etc.

I tested with Win7, Mac OS X 10.6.8 and Linux (OpenBox). It happens always.