Further information...
The PageDockSite XAML below shows a ToolItemsSource binding to Displays. During a drag operation from one PageDockSite to another the source decrements the Displays collection, but the target doesn't change?
<local:PageDockSite x:Name="PageDockSite"
ToolItemsSource="{Binding Displays}"
ToolItemContainerStyle="{StaticResource DisplayStyle}"
ToolWindowsHaveTabImages="False"
CanToolWindowsAttach="True"
CanToolWindowsBecomeDocuments="False"
CanToolWindowsDragToLinkedDockSites="True"
CanToolWindowsFloat="True"
CanToolWindowsDock="True"
CanToolWindowsClose="True"
CanToolWindowsAutoHide="False"
IsTabLayoutAnimationEnabled="False"
IsDockGuideAnimationEnabled="False"
IsLiveSplittingEnabled="True"
UseLayoutRounding="True"
SplitterSize="3"
Padding="1,1,1,1"
AreNewTabsInsertedBeforeExistingTabs="False"
AutoHidePopupOpenAnimationDuration="0"
AutoHidePopupCloseAnimationDuration="0"
FloatingToolWindowContainersHaveMaximizeButtons="True"
SerializationId="{Binding Path=ScopeIdentity.Identity}">
<local:PageDockSite.Switcher>
<docking:StandardSwitcher AreDocumentsVisible="False" />
</local:PageDockSite.Switcher>
</local:PageDockSite>
</Grid>
