RadSplitter is loaded with the gray background when configured with Default/Silks/Bootstrap skins in RenderMode="Classic".

The issue applies to RadFileExplorer and RadEditor file browser dialogs too which use RadSplitter inside.

The background is white in version 2017.1.118.40 and earlier.

Workaround:
Switch to RenderMode="lightweight"

or 

RadSplitter: Apply the BackColor property to RadSplitter or its RadPane items.
RadFileExplorer: 
ASPX:
<telerik:RadFileExplorer ID="RadFileExplorer1" runat="server" RenderMode="Classic">
    <Configuration ViewPaths="~/"  />
</telerik:RadFileExplorer>
Codebehind:
using Telerik.Web.UI;

public partial class Default2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        (RadFileExplorer1.Splitter.GetPanes()[0] as RadPane).BackColor = System.Drawing.Color.White;
        (RadFileExplorer1.Splitter.GetPanes()[1] as RadPane).BackColor = System.Drawing.Color.White;
    }
}

Visual regressions in RadSplitter:

    [Lightweight] [IE-all] [All-Skins]  vertical sliding panes tab text is upside down now: https://www.screencast.com/t/GhdIru4Od2sU
    [Classic] [all-browsers] [BlackMetroTouch,MetroTouch] - missing SlidingPane icons - https://www.screencast.com/t/qc18E9vU7MSF
    [Classic] [all-browsers] [Metro, MetroTouch] - missing backgrounds in the entire splitter: https://www.screencast.com/t/MBjdwph1wcW
    [Classic] [all-browsers] [All-Skins] - missing active sliding pane effect (expected: https://www.screencast.com/t/lFAnFdyzwNRU , actual: https://www.screencast.com/t/jyDdWJHIa10 )
    [Classic] [all-browsers] [Silk] - missing sliding pane header background: https://www.screencast.com/t/Nrk33Jokvuao

Workaround is to use the Lightweight RenderMode as is affected in the least, or roll back to R3 2016 SP2

The print preview resulted by the Splitter's print() method appears blank in Chrome in case there are CSS files passed as an argument to method.

Steps to reproduce:
1. Open http://demos.telerik.com/aspnet-ajax/splitter/examples/printpane/defaultcs.aspx in Chrome
2. Click in the "Print LeftPane" button


Result: An empty print-preview window is shown.

Potential workaround is attached but it requires modification of the markup to add a class to the pane that holds the vertical sliding zone as well. If you do not need this fixed urgently, wait for an official fix.

The client events of RazSlidingZone are not triggered properly when the zone is placed in a nested Splitter. For example, when a Sliding pane is docked the triggered events are ClientDocking (2 times) and ClientUndocking (1 time) instead of one time ClientDocking.


Code to reproduce the problem:
Note: Removing the decked pane of the parent splitter leads to proper behavior.

        <telerik:RadSplitter ID="MainSplitter" runat="Server" Width="100%" Height="100%">
            <telerik:RadPane ID="ViewerDocumentsPane" runat="Server">
                <telerik:RadSlidingZone ID="ViewerDocumentsSlidingZone" runat="Server"
                    DockedPaneId="ViewerDocumentsSlidingPane">
                    <telerik:RadSlidingPane ID="ViewerDocumentsSlidingPane" runat="Server" Title="Lef pane">
                    </telerik:RadSlidingPane>
                </telerik:RadSlidingZone>
            </telerik:RadPane>

            <telerik:RadPane ID="ViewerMainContainerPane" runat="Server">
                <telerik:RadSplitter ID="ViewerMainContainerSplitter" runat="Server"
                    Orientation="Horizontal">
                    <telerik:RadPane ID="ViewerInvalidSignaturePane" runat="Server">
                    </telerik:RadPane>

                    <telerik:RadPane ID="ViewerDocumentInfoPane" runat="Server" Height="22px">
                        <telerik:RadSlidingZone ID="ViewerDocInfoSlidingZone" Height="22px"
                            SlideDirection="Top"
                            runat="Server" DockedPaneId="ViewerDocInfoPane">
                            <telerik:RadSlidingPane Title="slidingPane"
                                ID="ViewerDocInfoPane" runat="Server"
                                OnClientDocking="OnClientDocking"
                                OnClientUndocking="OnClientUndocking">
                            </telerik:RadSlidingPane>
                        </telerik:RadSlidingZone>
                    </telerik:RadPane>

                </telerik:RadSplitter>
            </telerik:RadPane>

            <telerik:RadPane ID="ViewerDocumentDetails" runat="Server">
            </telerik:RadPane>
        </telerik:RadSplitter>

        <script>
            function OnClientDocking(sender, eventArgs) {
                console.log("OnClientDocking");
            }

            function OnClientUndocking(sender, eventArgs) {
                console.log("OnClientUndocking");
            }
        </script>

The text of the tabs inside a SlidingZone is not displayed properly when the SlidingZone is updated via AJAX. 

Steps to reproduce:
Run the following code and click a button inside a SlidingPane.

<telerik:RadSplitter ID="RadSplitter1" runat="server">
<telerik:RadPane ID="RadPane1" Scrolling="Both" Width="30px" runat="server">
<telerik:RadSlidingZone ID="RadSlidingZone1" runat="server" Width="30px">
<telerik:RadSlidingPane ID="RadSlidingPane1" runat="server" Title="Menu 1" Width="280px">
<telerik:RadButton runat="server" ID="btn1" AutoPostBack="true" Text="Update SlidingZone"></telerik:RadButton>
</telerik:RadSlidingPane>
<telerik:RadSlidingPane ID="RadSlidingPane2" runat="server" Title="Side Menu Two" Width="280px">
<telerik:RadButton runat="server" ID="btn2" AutoPostBack="true" Text="Update SlidingZone"></telerik:RadButton>
</telerik:RadSlidingPane>
</telerik:RadSlidingZone>
</telerik:RadPane>
<telerik:RadPane ID="AdditionalPane1" runat="server"></telerik:RadPane>
</telerik:RadSplitter>
<telerik:RadAjaxManager runat="server" ID="RadAjaxManager">
<AjaxSettings>
<telerik:AjaxSetting AjaxControlID="RadSlidingZone1">
<UpdatedControls>
<telerik:AjaxUpdatedControl ControlID="RadSplitter1" />
</UpdatedControls>
</telerik:AjaxSetting>
<telerik:AjaxSetting AjaxControlID="RadSlidingZone1">
<UpdatedControls>
<telerik:AjaxUpdatedControl ControlID="RadSplitter1" />
</UpdatedControls>
</telerik:AjaxSetting>
</AjaxSettings>
</telerik:RadAjaxManager>

If the title of a SlidingPane is more than 22px it gets truncated in vertically oriented Splitter.


The issue is reproducible in Edge

http://demos.telerik.com/aspnet-ajax/splitter/examples/sp_firstlook/defaultcs.aspx

Workaround:
 <style type="text/css">

        .rspPaneTabContainer{
            height: initial !important;
        }

    </style>

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>

When all Splitter's RadPanes have height configured in pixels, the control cannot be resized with the browser even if its width is set to 100%.

Steps to reproduce:
1. Run the following code and try to resize the width of the browser:
        <telerik:RadSplitter ID="VisualSplitter" runat="server" Orientation="Horizontal" Height="600px" Width="100%">
            <telerik:RadPane ID="TNPane" runat="server" Height="200" BackColor="WhiteSmoke" ContentUrl="Grid.aspx">
            </telerik:RadPane>
            <telerik:RadPane ID="ICPane" runat="server" Height="400" Scrolling="None" BackColor="Red" >
            </telerik:RadPane>
        </telerik:RadSplitter>


Temporary workaround:
Set the sum of all RadPanes' height as height of the Splitter and remain one RadPane's height not configured (the Splitter will calculate it on its own):
        <telerik:RadSplitter ID="VisualSplitter" runat="server" Orientation="Horizontal" _Height="600px" Width="100%">
            <telerik:RadPane ID="TNPane" runat="server" Height="200" BackColor="WhiteSmoke" ContentUrl="Grid.aspx">
            </telerik:RadPane>
            <telerik:RadPane ID="ICPane" runat="server" Scrolling="None" BackColor="Red" >
            </telerik:RadPane>
        </telerik:RadSplitter>

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

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

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 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

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

 The title of the SlidingPane is truncated in IE8 - http://screencast.com/t/yiJCR8DRp

Workaround:

        .rspSlideContainer td.rspSlideTitleContainer div.rspSlideTitle
        {
            width: 100% !important;
        }

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.

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.