Gimp 2.8.10 on Ubuntu 14.10

I have had a stable single-window configuration of Gimp for quite some time. Something happened and a dock got moved in the single window mode and I have never discovered any way to reconfigure the docks in this mode.

My preferred configuration is to have a single column of docks on the left of the main drawing area with the toolbox on top, the tool options below that, and the layers/paths/etc. tab dock on the bottom. My notes describing how to accomplish this say that the method is to:

1: disable single-window mode 2: configure these three docks as desired 3: stack them up as described with the main window on the right 4: reset single-window mode

The single window is supposed to assume this configuration, but it no longer does. Instead, it rearranges the three docks as three adjacent vertical columns on the left.

It is doubly maddening because I have saved the previous window layout with Edit->Preferences->Window Mgmt->Save Window Position Now, but somehow, Save window position on exit got checked and the saved configuration was lost.

Can anyone please explain what the trick is in order to get back to my previous configuration? Thanks.
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Jeff