WPF: Difference between binding and real value when it comes to rendering - wpf

Question

I'm working on a rather big project, that's why I can't really paste working code, but maybe you can give me a hint on what's wrong from reading the symptoms. Otherwise I have to see how I can re-create the problem in a smaller project to paste it in here.
I have a custom control that inherits from TreeView, let's call it MyTreeView. This control is using its own containers, let's call them MyTreeViewItems.
The whole thing is styled and tweaked that it looks like a family tree or an org chart (horizontally instead of vertically, parents on top, children below). The connecting lines are drawn in the OnRender procedure of MyTreeView.
Protected Overrides Sub OnRender(drawingContext As DrawingContext)
Dim bounds As ItemBounds = Nothing
If (Me.ConnectorPen IsNot Nothing) Then
Me.GetItemBounds(Me, bounds)
Me.DrawConnections(Me.ConnectorPen, drawingContext, bounds)
End If
MyBase.OnRender(drawingContext)
End Sub
ConnectorPen is a dependency property of type Pen and ItemBounds is a small hierarchical helper class holding the bounding rectangles of the content of all MyTreeViewItems.
GetItemBounds collects the bounding rectangles of the content of every MyTreeViewItem.
Private Sub GetItemBounds(parent As ItemsControl, ByRef bounds As ItemBounds)
Dim tvi As MyTreeViewItem
Dim topLeft As Point
Dim bottomRight As Point
Dim current As ItemBounds = Nothing
For Each item As Object In parent.Items
tvi = TryCast(parent.ItemContainerGenerator.ContainerFromItem(item), MyTreeViewItem)
If (tvi IsNot Nothing) Then
topLeft = tvi.Content.TranslatePoint(New Point(0, 0), Me)
bottomRight = tvi.Content.TranslatePoint(New Point(tvi.Content.ActualWidth, tvi.Content.ActualHeight), Me)
If (bounds Is Nothing) Then
bounds = New ItemBounds
bounds.Bounds = New Rect(topLeft, bottomRight)
current = bounds
Else
current = New ItemBounds
current.Bounds = New Rect(topLeft, bottomRight)
bounds.Children.Add(current)
End If
End If
Me.GetItemBounds(tvi, current)
Next item
End Sub
tvi.Content is the ContentPresenter holding the content of the current item.
DrawConnections draws the connecting lines according to the collected item bounds:
Private Sub DrawConnections(p As Pen, drawingContext As DrawingContext, bounds As ItemBounds)
Dim minChildTop As Double
Dim middle As Double
If (bounds Is Nothing) OrElse (bounds.Children Is Nothing) OrElse (bounds.Children.Count = 0) Then
Exit Sub
End If
If (bounds.Children.Count > 1) Then
minChildTop = bounds.Children.Min(Of Double)(Function(x As ItemBounds) x.Bounds.Top)
middle = bounds.Bounds.Bottom + ((minChildTop - bounds.Bounds.Bottom) / 2)
drawingContext.DrawLine(p, bounds.Bounds.BottomCenter,
New Point(bounds.Bounds.BottomCenter.X, middle))
drawingContext.DrawLine(p, New Point(bounds.Children.First.Bounds.TopCenter.X, middle),
New Point(bounds.Children.Last.Bounds.TopCenter.X, middle))
End If
For Each item As ItemBounds In bounds.Children
If (bounds.Children.Count > 1) Then
drawingContext.DrawLine(p, New Point(item.Bounds.TopCenter.X, middle),
item.Bounds.TopCenter)
Else
drawingContext.DrawLine(p, bounds.Bounds.BottomCenter,
item.Bounds.TopCenter)
End If
Me.DrawConnections(p, drawingContext, item)
Next item
End Sub
BottomCenter and TopCenter are extension methods for Rect to determine the Point which cuts the top edge or the bottom edge into two same-length halves.
Everything works fine so far. The space between one parent item and it's children is created by a fix margin that is added to the StackPanel, that is used as the ItemsPanel in the default style of MyTreeViewItem.
<Style TargetType="{x:Type local:MyTreeViewItem}" BasedOn="{StaticResource {x:Type TreeViewItem}}">
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal"
HorizontalAlignment="Center"
IsItemsHost="True"
Margin="0,20,0,0" />
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
...
</Style>
Now the customer wants this space between parent and children to be changeable from outside the control. So I added a dependency property called ItemSpacing of type Double to MyTreeView, and changed the default style of MyTreeViewItem to use this property instead of the fixed margin (conversion from Double to Thickness is done by a converter).
<Style TargetType="{x:Type local:MyTreeViewItem}" BasedOn="{StaticResource {x:Type TreeViewItem}}">
<Setter Property="ItemsPanel">
<Setter.Value>
<ItemsPanelTemplate>
<StackPanel Orientation="Horizontal"
HorizontalAlignment="Center"
IsItemsHost="True"
Margin="{Binding ItemSpacing, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MyTreeView}}, Converter=..." />
</ItemsPanelTemplate>
</Setter.Value>
</Setter>
...
</Style>
Now the child items use the space provided by ItemSpacing (so that works), but the drawing of the connecting lines fails. It looks like the collecting of the bounding rectangles of the content of all MyTreeViewItems is done before this bound margin is applied.
I already tried to play around with the AffectsRender and AffectsArrange settings of the ItemSpacing dependency property, but that didn't help.
Can you tell me, why this is working with a fix margin and why it isn't working when the margin comes from a binding?