It might be the theme as well. I struggle a bit with that piece of source code, I see at your site:
<aside id="featured_category_widget-2" class="widget widget_featured_category_widget mCustomScrollbar _mCS_1"><div class="mCustomScrollBox mCSB_horizontal" id="mCSB_1" style="position:relative; height:100%; overflow:hidden; max-width:100%;"><div class="mCSB_container" style="position: relative; left: -0.483333px; width: 4752px;"><div id="featuredimg">
Where does it come from? I don’t use aside, I use div as far as I remember. And there seems to be a container inside the widget, which I never have. Also, it seems to float, which my widget doesn’t either. I will have to have a closer and longer look, to help you, I guess. Anyway, the file that you want to check is: category-feature/class-lib/CF-WidgetClass.php
Regards