When using commandbars with dockingpanes, commandbars will handle all resizing events for you. (DockingPane.SetCommandBars Me.CommandBars)

 

also the ResizeClient event.

so you could determine what space you have left for the treeview like this:

 

Private Sub CommandBars_ResizeClient(ByVal Left As Long, ByVal Top As Long, ByVal Right As Long, ByVal Bottom As Long)
   

    If StatusBar1.Visible Then Bottom = StatusBar1.Top
    TreeView1.Move Left, Top, Right - Left, Bottom - Top

End Sub

 

Docking the panes at startup:

I haven't found a way to lock the client area on a certain position, maybe it's possible, maybe not. So, docking the panes to the right side of the form will position the client area on the left side. If you want the second pane below the first one you can use the last param to dock that pane as neighbour:

 

Dim Pane

Set Pane = DockingPane.CreatePane(1, 400, 200, DockRightOf, Nothing)

 

Dim Pane2 As Pane
    Set Pane2 = DockingPane.CreatePane(2, 400, 200, DockBottomOf, Pane) ''Pane is the pane you already created

 

I would use the statusbar of commandbars so you don't have to deal with size and position etc. when dragging toolbars to the bottom of the form. Test this and you will see what I mean. 

 

 

The second issue that I thought I was having (and which is the subject of this post) was that if you click Panel A in the tree view control a panel appears docked to the right side of the main form. I have since determined that the black border around the form in this panel is the result of having a hidden menu on the form. However, in this sample project, the height of the panel is correct both when it is intially displayed and hidden/unhidden. I guess I must have something different in my 'real' project - which is too large to send - that is causing the problem I am having