Hi @anniegronow,
You can sort of do this by using a group stuck to the page top similar to @duncan’s suggestion.
Create your two tabs, add a third box with no fill (transparent) at the top of the canvas (effectively the top of the browser window). Group all three tabs and make the group stick to browser top with overlap.
Looks like this:
This seems to work as you would wish. The red and green tabs appear to scroll down but in reality the page content scrolls behind them.