Hello Telerik Team,

I'm implementing HeaderContext filtering with checkbox lists on a RadGrid in a legacy ASP.NET WebForms application. 

Current Setup:
- FilterType="HeaderContext"
- FilterCheckListEnableLoadOnDemand="true"
- OnFilterCheckListItemsRequested event populates checkboxes from session data

Issue:
Users experience 500-2000ms delay when opening filter menus (AJAX callback to server). 
In our production environment with slow network and high concurrent users, this creates 
poor user experience.

Questions:
1. Is there a way to pre-populate checkbox lists during grid initialization 
   (avoiding AJAX callback)?
2. Can we use OnItemCreated to populate checkbox lists with 
   FilterCheckListEnableLoadOnDemand="false"? If so, what's the correct approach?
3. Is there a client-side caching mechanism for checkbox list items?
4. What's the recommended pattern for large datasets (1000+ rows) with multiple 
   filterable columns?

Environment:
- Telerik.Web.UI version: v4.0.30319
- .NET Framework: 4.8
- Browser: Chrome, IE11
- Grid has ~10 columns with 5 using checkbox filters

Any guidance on performance optimization would be greatly appreciated!

Thank you,
Anurag pandey

After upgrading to Telerik UI for ASP.NET AJAX 2026 Q1 (2026.1.211.462) the HtmlChart Sparkline cannot initialize.

The demo page loads, but the chart is empty and the component is not usable.

The browser console shows a JavaScript initialization failure originating from the Telerik scripts.

The first error thrown is:

Uncaught ReferenceError: kendo is not defined
    at Telerik.Web.UI.WebResource.axd...

Immediately after that a second error appears:

Uncaught TypeError: kendo.jQuery(...).kendoSparkline is not a function

The second error is a cascade failure because the Kendo widgets are never registered due to the first error.

The gantt throws Uncaught ReferenceError: kendo is not defined when clicking on the calendar picker button.

The spreadsheet throws Uncaught ReferenceError: kendo is not defined when you type in a number in the quantity field, then click to the next cell.

For a better description of the issue and reproduction steps, please view the ticket:

http://www.telerik.com/account/support-tickets/view-ticket?threadid=1010438

If a user clicks 'Ignore All' for a misspelled word, the focus moves to the next misspelled word after the last word included in the ignore all list.  This becomes problematic when editing a large quantity of text and the user's trying to go through things in order, but the focus moves to the very bottom of the content area.

In MS Word, it works as our users expect it would.  Once 'Ignore All' is chosen, the very next misspelled word is highlighted.

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 MaxFileSize property is an int - this means a limit over 2GB can't be set. Please change this property to a long. 

A function that is attached to this event is triggered on clicking the RadWindow's toolbar, even if it is already active. 

In lightweight the issue is not observed:

<telerik:RadRibbonBar runat="server" RenderMode="Classic">
    <Tabs>
        <telerik:RibbonBarTab Text="home">
            <%-- duplicate the group and items inside to replicate more easily --%>
            <telerik:RibbonBarGroup Text="Documents">
                <Items>
                    <telerik:RibbonBarButton Text="Item 1" />
                </Items>
            </telerik:RibbonBarGroup>
        </telerik:RibbonBarTab>
    </Tabs>
</telerik:RadRibbonBar>

When the height of FileExplrorer is bigger that the current view port and an item from its right pane is selected, the content of the page jumps (the active element is brought to the top corner of the page).

video: http://screencast.com/t/Ezuteb3xIoCp