The RadComboBox in classic render mode uses background image for displaying borders and background. The reason is supporting the controls also in browsers that does not support rounded corners and gradients. If you set wider width to the expand button with arrow, the other parts of the sprite becomes to be visible.
To fix the issue, please remove the width property of the button, or use lightweight render mode
Please, keep in mind, that all RadComboBox used on a one web page have to be in the same render mode.
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