The tabs get hidden when a top oriented SlidingPane is docked inside a horizontal SlidingZone. The issue is reproducible when the resize mode of the parent Splitter is != AdjacentPane.

Steps to reproduce:
1. Run the following code:

        <div style="padding: 200px">
            <telerik:RadSplitter runat="server" ID="RadSplitter2" Orientation="Horizontal" ResizeMode="Proportional">
                <telerik:RadPane runat="Server" ID="radpane11" Height="20px" MaxWidth="980" Scrolling="None" BackColor="Green">
                    <telerik:RadSlidingZone ID="ListMetricsSlidingZone" runat="server" Height="25px" DockedPaneId="ListMetricsPane" SlideDirection="Top">
                        <telerik:RadSlidingPane ID="ListMetricsPane" Title="DEF" runat="server">
                            1111
                        </telerik:RadSlidingPane>
                        <telerik:RadSlidingPane ID="ListHistorySlidingPane1" Title="XYZ" Font-Size="X-Large" runat="server">
                            222
                        </telerik:RadSlidingPane>
                    </telerik:RadSlidingZone>
                </telerik:RadPane>
                <telerik:RadPane runat="Server" ID="radpane33" Scrolling="None">
                </telerik:RadPane>
            </telerik:RadSplitter>
        </div>

2. Undock and dock a sliding pane.

Expected: The tabs remains visible all the time (change the Splitters ResizeMode="AdjacentPane" for reference)