iceshrimp-legacy/packages/client/src/scripts/sticky-sidebar.ts

export class StickySidebar {
	private lastScrollTop = 0;
	private container: HTMLElement;
	private el: HTMLElement;
	private spacer: HTMLElement;
	private marginTop: number;
	private isTop = false;
	private isBottom = false;
	private offsetTop: number;
	private globalHeaderHeight = 59;

	constructor(
		container: StickySidebar["container"],
		marginTop = 0,
		globalHeaderHeight = 0,
	) {
		this.container = container;
		this.el = this.container.children[0] as HTMLElement;
		this.el.style.position = "sticky";
		this.spacer = document.createElement("div");
		this.container.prepend(this.spacer);
		this.marginTop = marginTop;
		this.offsetTop = this.container.getBoundingClientRect().top;
		this.globalHeaderHeight = globalHeaderHeight;
	}

	public calc(scrollTop: number) {
		if (scrollTop > this.lastScrollTop) {
			// downscroll
			const overflow = Math.max(
				0,
				this.globalHeaderHeight +
					(this.el.clientHeight + this.marginTop) -
					window.innerHeight,
			);
			this.el.style.bottom = null;
			this.el.style.top = `${
				-overflow + this.marginTop + this.globalHeaderHeight
			}px`;

			this.isBottom =
				scrollTop + window.innerHeight >=
				this.el.offsetTop + this.el.clientHeight;

			if (this.isTop) {
				this.isTop = false;
				this.spacer.style.marginTop = `${Math.max(
					0,
					this.globalHeaderHeight +
						this.lastScrollTop +
						this.marginTop -
						this.offsetTop,
				)}px`;
			}
		} else {
			// upscroll
			const overflow =
				this.globalHeaderHeight +
				(this.el.clientHeight + this.marginTop) -
				window.innerHeight;
			this.el.style.top = null;
			this.el.style.bottom = `${-overflow}px`;

			this.isTop =
				scrollTop + this.marginTop + this.globalHeaderHeight <=
				this.el.offsetTop;

			if (this.isBottom) {
				this.isBottom = false;
				this.spacer.style.marginTop = `${
					this.globalHeaderHeight +
					this.lastScrollTop +
					this.marginTop -
					this.offsetTop -
					overflow
				}px`;
			}
		}

		this.lastScrollTop = scrollTop <= 0 ? 0 : scrollTop;
	}
}