> "it would be useful for mobile browsers to allow adding temporary scroll bookmarks while using a page"
This shouldn't be necessary with a well-designed menu interface. Firstly the whole thing should be indexed anyway, allowing easy jumping between categories. And if something looks good you should be able to favourite it with a single tap, or at least add it to your order with a single tap. (If you end up with too many items in you order, consider it a short-list, reviewing the items and narrowing it down from the final order review page).
I'm thinking more in terms of browsing, not just selectable items in a menu, but any scrollable content.
A real-world analog would be adhesive color tabs people stick to the sides of books or printed documents to mark a paragraph non-destructively, so they can return to that point later more easily.
Think anchor headings, except user-driven, because many websites don't even use anchors to enable linking to page sections by URL hashes, and without inspecting headings for anchors (by tapping or long-pressing on them), nobody would ever know it's a feature.
I wish I could just mark my current scroll position and return to it later by navigating back + forth between the ones I've saved for a page. I've lost count how many times I've made mental notes saying "This is interesting, I'll return to this bit later" only to struggle finding it again because it's lost in a sea of text. Browsers have no way to temporarily bookmark bits of content without developers (or CMS'es) being smart enough to anchor headings and sections.
Indexing is INHERENT to a 2D document/file. It doesn't need to be indexed or categories. It can hold ALL the information you want to peruse and compare in a given moment. You just, simply look around and view it! - willy wonka
This shouldn't be necessary with a well-designed menu interface. Firstly the whole thing should be indexed anyway, allowing easy jumping between categories. And if something looks good you should be able to favourite it with a single tap, or at least add it to your order with a single tap. (If you end up with too many items in you order, consider it a short-list, reviewing the items and narrowing it down from the final order review page).