How I detected a Shopify app that was breaking checkout only on mobile

I'm sharing a field experience in DTC & Shopify stores. I had been working with a DTC store with a custom theme, upsell apps, and traffic primarily from mobile when some customers appeared who couldn't finish checkout from mobile Safari. The symptom that made me investigate it seriously was that desktop worked, analytics showed a drop-off, and support received screenshots with no clear error. I…