1. Run the Demo Application 
2. Click the "Dock" item on the left list
3. Click "Programming" tile then lunch the example
4. Drag and drop the Tool Window 1 and/or Tool Window 2 to the document window 1 area to set Tool Window 1 and/or Tool Window 2 as tabbed document
5. Then you can only click and select Tool Window 1 and/or Tool Window 2, but you can't click and select document window 1

Resolution: 
In the Dock => Programming example is added code which prevent selection: 
private void radDock1_ActiveWindowChanging(object sender, DockWindowCancelEventArgs e)
{
	e.Cancel = e.NewWindow is DocumentWindow;
}

We modified the example to log information when select different windows.