I ended up modifying source of AvalonDock because it's just addition of 6 lines. If you know any better way, please post your answer.
In switch
statement of FilterMessage
method of LayoutAnchorableFloatingWindowControl.cs
case Win32Helper.WM_NCLBUTTONDBLCLK:
_model.Descendents().OfType<LayoutAnchorablePane>().First(p => p.ChildrenCount > 0 && p.SelectedContent != null).SelectedContent.Dock();
break;
In switch
statement of FilterMessage
method of LayoutDocumentFloatingWindowControl.cs
case Win32Helper.WM_NCLBUTTONDBLCLK:
_model.RootDocument.Dock();
break;