>>808のと質問内容は違うけど、本質は似たようなものだから試してみればいい
俺は試してないから確証は持たないけど

ttp://forum.xfce.org/viewtopic.php?id=7263
>Open GIMP -> edit -> preferences -> window management -> change dropdowns
>for "Hint for the toolbox:" and "Hint for other docs:" to "Normal window" or "keep above". Then restart GIMP.
>They should now stay in the same workspace as the main window.