Hi there!
Thanks for your query.
Your header columns needs to be resized.
However, you may reduce the font size of your menu items to fit then one line. Here is a sample code snippet you need to use in your custom CSS:
.sp-megamenu-parent>li>a, .sp-megamenu-parent>li>span {
font-size: 10px;
}
Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
Could not understand what to close to see the jumping issue, kindly share an screencast to get it better.
Best Regards