Reproduces the Docebo / ivalua downloadCourseFile flow: a click opens a GENUINELY blank tab (about:blank), then that tab is navigated to the download in a SECOND step on a timer, so the download navigation is NOT tied to the click's user gesture (has_user_gesture=f). The target 302-redirects twice before landing on the S3-style attachment. In Isolation the surrogate opens the about:blank tab, the navigation resolves to a download instead of a page (surrogate-nav-not-committed), and the tab is torn down.