Preloading just before visibility using rootMargin, thresholds, and a stability check

A common lazy-loading dilemma is timing: waiting until an element is visible causes visible pop-in, while starting too early wastes bandwidth. The described approach separates three concerns: when to detect proximity, which visibility threshold counts, and how to make the load decision stable. A positive bottom rootMargin begins fetching while the element is still approaching the viewport, and a WeakMap flag ensures the load runs only once per element. A debounced re-check of the element's dimensions and a predicate guards against layout jitter from reflow or resizing. The author notes that the pattern validates the decision logic but cannot simulate real browser geometry, network timing, or device scrolling behavior, so rootMargin and thresholds must be tuned on supported devices. Caching, connection prioritization, and post-fetch handling are explicitly out of scope.