Preview themes on your own store and checkout by market
The code editor always saves into the theme you are editing
When the theme you were editing had no code files yet, such as a new development theme, the code editor could show and save files that belonged to another of your themes. It now only shows and saves the files of the theme you are editing. A theme opened in the code editor for the first time starts from that theme's own files.
Checkout opens in the market your customer was shopping in
Checkout now starts with the shipping country of the market your customer was browsing, so the prices in checkout match the product page and the cart. A saved checkout country or the default address of a signed-in customer still comes first. The shipping address decides prices and VAT: when your customer picks a country in another market, checkout shows the new prices and says "Prices updated for" that country. The embedded checkout follows the same rules.
Preview any theme on your own store
Every theme in your theme library now has "Preview on my store". It opens your real store in that theme, with your own products, menus and texts and the theme's default look and page layouts, so you see exactly what activating it would give you. A bar at the bottom lets you activate the theme, close the preview or copy the link. The link works for 24 hours, so you can share it with a colleague or designer. Development-theme links show your unpublished edits. Nothing changes for your customers until you activate the theme. Previewing your development theme now also opens it on your store instead of showing your live theme.