[Solved] RadWindow incorrectly calculates its size when containing a RadAjaxPanel

[Andreas]

Hello Telerik Support,

we recently upgraded our application from Telerik UI for ASP.NET AJAX 2025 Q2 to Telerik UI for ASP.NET AJAX 2026 Q2.

Since the upgrade, we have encountered an issue with the RadWindow control when it contains a RadAjaxPanel.

The height and width of the RadWindow are calculated incorrectly. As a result, the content inside the RadWindow is either cut off or the window is significantly larger or smaller than expected.

We were able to reproduce the same behavior in the official Telerik demo:

https://demos.telerik.com/aspnet-ajax/window/examples/autosize/defaultcs.aspx

We have attached a screenshot showing the incorrect behavior in the Telerik demo.

Steps to reproduce

1. Open the Telerik RadWindow AutoSize demo.
2. Use a RadAjaxPanel inside the RadWindow.
3. Compare the behavior between Telerik UI for ASP.NET AJAX 2025 Q2 and 2026 Q2.
4. With 2025 Q2, the window size is calculated correctly.
5. With 2026 Q2, the height and/or width is calculated incorrectly.

Expected behavior

The RadWindow should automatically calculate its size based on the actual dimensions of the content inside the window, including the content of the RadAjaxPanel.

Actual behavior

With Telerik UI for ASP.NET AJAX 2026 Q2, the RadWindow calculates an incorrect height and width when a RadAjaxPanel is present.

Could you please let us know:

• Is this a known issue in 2026 Q2?
• Was there a change in the RadWindow or RadAjaxPanel auto-sizing behavior between 2025 Q2 and 2026 Q2?
• Is there a recommended workaround or configuration we can use?
• Will this issue be fixed in an upcoming release?

Since the issue can also be reproduced in the official Telerik demo, we assume that this is related to a change in the Telerik controls rather than our application.

Thank you for your help.

Best regards