Hi Gregory,

Sharing my response from the private ticket here so it’s visible to other developers who might run into the same behavior.

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Based on your description and the attached materials, the erratic behavior of the TelerikComboBox inside a TelerikWindow when using the ContainmentSelector attribute is a known limitation. The ComboBox dropdown popup relies on specific positioning logic, and restricting it with ContainmentSelector can interfere with its rendering and interactivity. This is why removing the ContainmentSelector resolves the issue.

Key Points and Suggestions:

• The popup used by the ComboBox is designed to be rendered at the root level of the application. Applying ContainmentSelector limits the popup to a specific container, which can cause issues with opening, selection, and overall usability.

• If possible, avoid using the ContainmentSelector with popup-based components like ComboBox. If containment is required, you may need to consider alternative UI arrangements or input components that do not use popups.

• Double-check that the <TelerikRootComponent> wraps your entire application layout and that no parent elements have non-default positioning styles (such as position: relative, top, or left). These styles can also affect popup positioning. More details are available here: https://www.telerik.com/blazor-ui/documentation/troubleshooting/general-issues

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With that in mind, I’m marking this item as Declined, since it’s a documented limitation rather than a bug.

Regards,
Hristian Stefanov
Progress Telerik