How Do I Fix a Menu on Top When Scrolling?

For the Fixed Menu Widget

Click on the menu in the site header and switch on Fix the menu on top on the left panel.

A click on the "gear" icon opens background settings: here you can configure color and opacity.

For the Menu Widget

Place the Menu widget in a separate block at the top of the website adding only those widgets that you want to be pinned when scrolling. Then fix the menu block by clicking on the "3 dots" icon on the block settings panel and selecting Pinned block settings.

In the window that will open, enable Pin block and specify the top position. Read more about pinned block settings in the article “How Do I Create a Pinned Block?”