TL;DR
Vertical video feeds are fast on the first item and stutter on every one after. We'll build a preload window that follows scroll direction, attaches hls.js sources to off-screen items ahead of the user, and tears down everything outside the window so you stay under Safari's video element ceiling. Reconciler pattern, ~120 lines.
The problem
Item 1 plays instantly because the page loaded around it. Item 2 shows black, then a spinner, then a stutter into low quality. Preloading everything fixes the stutter and replaces it with a data-plan fire and a crash on iPhone.
What we want: preload a small number of items in the direction the user is scrolling, and aggressively release everything else.
