internal void TestDrop(IDockDragSource dragSource, DockOutlineBase dockOutline)
        {
            if (!dragSource.CanDockTo(this))
                return;

            Point ptMouse = MousePosition;

            HitTestResult hitTestResult = GetHitTest(ptMouse);
            if (hitTestResult.HitArea == HitTestArea.Caption)
                dockOutline.Show(this, -1);
            else if (hitTestResult.HitArea == HitTestArea.TabStrip && hitTestResult.Index != -1)
                dockOutline.Show(this, hitTestResult.Index);
        }
        internal void TestDrop(IDockDragSource dragSource, DockOutlineBase dockOutline)
        {
            if (VisibleNestedPanes.Count == 1)
            {
                DockPane pane = VisibleNestedPanes[0];
                if (!dragSource.CanDockTo(pane))
                    return;

                Point ptMouse = MousePosition;
                uint lParam = Win32Helper.MakeLong(ptMouse.X, ptMouse.Y);
                if (NativeMethods.SendMessage(Handle, (int)Msgs.WM_NCHITTEST, 0, lParam) == (uint)HitTest.HTCAPTION)
                    dockOutline.Show(VisibleNestedPanes[0], -1);
            }
        }