Browsers tend to zoom content erroneously, which results in various glitches as the one you observe.
The problem seems to be triggered by the following CSS styles:
You can try overriding one or more of the above values (use higher CSS specificity), however, a quick test on my side showed that no value seems to work well in all possible browser zoom levels. You could come up with a combination that works better though.
the Telerik team
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