While implementing a scroll-spy for a client, the page kept scrolling to the top.

Initially, I figured it had been caused by code I wrote but after removing all my code I quickly realized this was built-in behavior.

The function to remove is in setActiveMedia on line 47 of media-gallery.js in the assets folder of the theme.

if (!this.elements.thumbnails || this.dataset.desktopLayout === 'stacked') {
    activeMedia.scrollIntoView({behavior: 'smooth'});
}

Comment this line out and it will no longer scroll automatically when a new variant is selected.

The function commented out.

This worked for me in Dawn 3.0.0 – 5.0.0 and likely works above those versions as well.