The tabs of SlidingPanes are not rendered (sized) properly is the SlidingPane is containing a nested Splitter. The issue is not reproducible with 2014.2.724 version of the controls.

Reproduction code:
        <telerik:RadSplitter ID="RadSplitter2" runat="server" Skin="Metro">
            <telerik:RadPane ID="RadPane1" runat="server" Width="22px">
                <telerik:RadSlidingZone ID="RadSlidingZone1" runat="server" Width="22px">
                    <telerik:RadSlidingPane ID="RadSlidingPane2" Title="What If" runat="server">
                        <telerik:RadSplitter ID="WhatIfPaneSplitter" runat="server" Width="50px">
                            <telerik:RadPane ID="RadPane2" runat="server"></telerik:RadPane>
                            <telerik:RadPane ID="RadPane3" runat="server"></telerik:RadPane>
                        </telerik:RadSplitter>
                    </telerik:RadSlidingPane>
                </telerik:RadSlidingZone>
            </telerik:RadPane>
            <telerik:RadPane ID="RadPane4" runat="server"></telerik:RadPane>
        </telerik:RadSplitter>

Hovering the tab of a SlidingPane will render the pane only the first time, after the second hovering only the title of the SlidingPane will be rendered. The issue is reproducible both in enabled and disabled ClickToOpen, in IE8.

Video: http://screencast.com/t/jFhmqoJR

When a Splitter contains both RadPanes with size in percent and free-sized panes, the width of the fomer is calculated properly only after the initial loading and remains the same the the page is resized.

Steps to reproduce:
1.     <telerik:RadSplitter ID="RadSplitter1" runat="server" Width="100%">
            <telerik:RadPane ID="RadPane1" runat="server" Width="10%"></telerik:RadPane>
            <telerik:RadSplitBar ID="RadSplitbar1" runat="server" ></telerik:RadSplitBar>
            <telerik:RadPane ID="RadPane2" runat="server"></telerik:RadPane>
         </telerik:RadSplitter>

2. Resize the browser.

Result: the size of RadPane1 is not recalculated

Temporary workaround:
Set size in perncent to all RadPanes in the Splitter.

A JavaScript error is thrown when a Splitbar is dragged in IE11 Edge mode and resolution 2560x1080. The issue is reproducible only when the ScriptManager's EnableScriptCombine property is set to False.

Temporary workaround: Set the RadScriptManager's EnableScriptCombine property to "True"

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)

The height of a  free-sized RadPane is not calculated properly on layout mode change in Android devices. The issue is reproducible in Firefox for Android.

If the title of a SlidingPane is more than 22px it gets truncated in vertically oriented Splitter. The issue is reproducible in IE11 and IE8 here:
http://demos.telerik.com/aspnet-ajax/splitter/examples/sp_firstlook/defaultcs.aspx

When a Tab of a Vertically oriented Splitter have both title and icon, its height is not calculated properly (the height of the icon is not taken into account). The issue is reproducible in Chrome.

Screenshot: http://screencast.com/t/0aaMryaShXZe

When a RadPane with not configured fixed size is locked its size is not calculated properly (and the size of all its sibling free-sized panes).The issue can be reproduced with the following code in Firefox (both in horizontally or vertically oriented Splitter):

    <telerik:RadSplitter ID="TestSplitter" runat="server" Width="100%" Height="100%"
        Orientation="Horizontal">
        <telerik:RadPane ID="RadPane1" runat="server"
            Locked="true">
        </telerik:RadPane>
        <telerik:RadPane ID="RadPane2" runat="server">
        </telerik:RadPane>
        <telerik:RadPane ID="RadPane3" runat="server">
        </telerik:RadPane>
    </telerik:RadSplitter>

These overrides can be used as a temporary workaround:

        Telerik.Web.UI.SplitterPaneBase.prototype.get_height = function () {
            return this._height === "" ? 0 : parseInt(this._height);
        }
        Telerik.Web.UI.SplitterPaneBase.prototype.get_width = function () {
            return this._width === "" ? 0 : parseInt(this._width);
        }

The HTML5 related changes in the RadSplitter have specific selectors that influence the content inside.

When RadSplitter is declared in a page with no Doctype the RadSlidingPane stops responding in IE and the layout looks broken when docking/undocking.

When the content of a RadPane is set through its ContentURL property, the RadPane cannot be scrolled in order to how its full content in IPAD.

The issue could be reproduced here: http://demos.telerik.com/aspnet-ajax/splitter/examples/externalcontentloading/defaultcs.aspx