In-app browser
An in-app browser is the stripped down browser embedded inside another application, which is where a purchase referred by an assistant frequently completes rather than in the shopper’s usual browser.
It differs from an ordinary browser in ways that matter to a checkout: no cookies, no saved address, no signed in state, often restricted storage, and different handling of redirects. Every session is a first session.
The narrow viewport is the least interesting difference and the one most testing covers. What actually breaks checkouts is a flow that stashes state client side across a redirect and cannot find it again on the return leg.
Why it matters to a store
A checkout that passes every device test can still fail the sessions an assistant sends, because those arrive in an environment ordinary mobile testing does not reproduce. That failure lands on the most valuable traffic you have: a shopper who has already chosen you.