Can reproduce this with MDI_DynamicBars sample.
Set the visual style to VS2005. This will enable the "docking guide" indicators when dragging a window to dock it.
Now then, float two of the dynamic bars. Place one over the center of the main application and set it to "Floating" if it isn’t already. Set the other window to "Dockable". Drag the dockable window towards the center of the application, and the four-way docking guide will appear over the other floating window. As the "dockable" window crosses over the "floating" window, however, the four-way guide disappears.
While I understand that the dockable window can’t dock to the floating window, the appearance and disappearance of the four-way guide seems confusing, particularly to users who may well assume they can utilize it to dock the window. If both are set to dockable, the four-way guide switches to allow the windows to dock in the same floating frame, which is fine. It is just this "floating" case that can get really confusing.
Bug? Feature enhancement? Not sure, but our testers found it, and I’m just passing it along. :)
K.