First determine if this problem is specific to your project or to this widget. If widget: contact the makers, but I suspect it is specific to your project. So let’s continue there: are you using Atlas-UI and no custom styling at all? Then maybe ask mendix at support.mendix.com. If it is related to the styling in the project, you best create a new one, use that widget where it probably will look ok and start comparing the styles in the browser inspector.