While changes to the styles of the controls are introduced with subsequent versions the goal is to improve them and we are doing our best to avoid breaking changes. However, we cannot predict all the possible custom CSS styles applied to a particular control and differences in the control's appearance when such styles are applied on different control versions are possible.
I would suggest opening a support ticket, in which you would be able to attach a sample project with the isolated ComboBox demonstrating the issue, because the forum does not allow attachments. This way we would be able to compare the behavior of the control using vQ2 2014 and vQ3 2015 and suggest a possible solution.
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