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 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.

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"

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.

The border of the splitbar disappears on hover. The issue is reproducible in IE - http://screencast.com/t/QfaZIZirSZfB

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

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

The scroll position of the radPanes is not pesisted after a postback (even if the PersistScrollPosition property of the Pane is set to "true").

When a RadPanelBar (having have more than 1 level of nested items) is nested inside a SlidingPane, it cannot be scrolled after expanding in iOS.

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

The border size of the RadPanes is always inherited by their parent Splitter's BorderSize, ragardless of the value set to the Splitter's PanesBorderSize property.

When a Sliding pane have to be resized over a RadPane, containing a heavy weight object (Silverlight, PDF, Flash, etc.) its size is not calculated properly.