The most probable cause for this appearance is a global CSS rule on the page that modifies the default margin/padding/size/etc. for elements like. TR, TD, UL, LI. Such rules are applied to all elements on the page, including the ones from the RadWindow, regardless of any other classes, ids and cascades there may be.
This means that having such global rules on your page will affect all third party controls, like ours. Such global rules should be avoided, as our controls rely on a clean CSS slate to work correctly. We could not override all CSS properties in our stylesheets, as this would increase their size many times and they would become unmaintainable.
How you can determine if this is the cause - disable (remove) all your custom CSS from the page and if the issue is gone you can start re-adding the files one by one until the issue manifests. Then the same process can be repeated for the rules inside to determine the offending one.
What you can do is make your selectors heavier, for example by adding a class to your rule. This would prevent it from applying to all other elements and thus breaking the display of third party controls.
the Telerik team
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