Some ideas!

aw-ideas.js

animationWorklet.import('twitter-header-animator.js').then( _ => {
	var anim = new WorkletAnimation('twitter-header', 
	 [document.timeline, new ScrollingTimeline(scrollingElement, {/* options for timeline */})],
	 {
	   targets : {
	      bar: element1,
	      avatar: [element2, element3],
	    },

	    timing: {
	    	duration: ....???
	    }
	});
}

//In animation worklet
registerAnimator('twitter-header', class {

	const easing = new EaseOutTiming();
	animate(timelines, targets) {
		const time = timelines[0].currentTime; // [0...timeRange]
		const progress = time / range; 

		targets.get("bar")[0].outputStuleMap.set("opacity", easing.value(progress))
		targets.get("avatar").forEach(avatar => {
			avator.outputStuleMap.set("opacity", progress);
		})
	}
});