AFTER: body overflow:auto — document scrolls Scroll this list on mobile. The address bar WILL collapse because the body itself overflows the viewport. This is what mobile browsers require to collapse their chrome.
/* Fixed: body is the scroll container */
body {
  overflow-y: auto; /* body scrolls — address bar can collapse */
}

/* No fixed child container needed */

Scroll down — address bar collapses

flutter/flutter #69529  ·  SO: iOS 15 address bar collapse