rudeshark.net/packages/client/src/directives/follow-append.ts

import { Directive } from "vue";
import { getScrollContainer, getScrollPosition } from "@/scripts/scroll";

export default {
	mounted(src, binding, vn) {
		if (binding.value === false) return;

		let isBottom = true;

		const container = getScrollContainer(src)!;
		container.addEventListener(
			"scroll",
			() => {
				const pos = getScrollPosition(container);
				const viewHeight = container.clientHeight;
				const height = container.scrollHeight;
				isBottom = pos + viewHeight > height - 32;
			},
			{ passive: true },
		);
		container.scrollTop = container.scrollHeight;

		const ro = new ResizeObserver((entries, observer) => {
			if (isBottom) {
				const height = container.scrollHeight;
				container.scrollTop = height;
			}
		});

		ro.observe(src);

		// TODO: 新たにプロパティを作るのをやめMapを使う
		src._ro_ = ro;
	},

	unmounted(src, binding, vn) {
		if (src._ro_) src._ro_.unobserve(src);
	},
} as Directive;