1 files changed, 43 insertions, 0 deletions
diff --git a/src/client/scripts/sticky-sidebar.ts b/src/client/scripts/sticky-sidebar.ts
new file mode 100644
index 0000000000..872e162d2b
--- /dev/null
+++ b/src/client/scripts/sticky-sidebar.ts
@@ -0,0 +1,43 @@
+export class StickySidebar {
+	private lastScrollTop = 0;
+	private el: HTMLElement;
+	private spacer: HTMLElement;
+	private marginTop: number;
+	private isTop = false;
+	private isBottom = false;
+
+	constructor(el: StickySidebar['el'], spacer: StickySidebar['spacer'], marginTop = 0) {
+		this.el = el;
+		this.spacer = spacer;
+		this.marginTop = marginTop;
+	}
+
+	public calc(scrollTop: number) {
+		if (scrollTop > this.lastScrollTop) { // downscroll
+			const overflow = this.el.clientHeight - window.innerHeight;
+			this.el.style.bottom = null;
+			this.el.style.top = `${-overflow}px`;
+
+			this.isBottom = (scrollTop + window.innerHeight) >= (this.el.offsetTop + this.el.clientHeight);
+
+			if (this.isTop) {
+				this.isTop = false;
+				this.spacer.style.marginTop = `${scrollTop}px`;
+			}
+		} else { // upscroll
+			const overflow = this.el.clientHeight - window.innerHeight;
+			this.el.style.top = null;
+			this.el.style.bottom = `${-overflow - this.marginTop}px`;
+
+			this.isTop = scrollTop <= this.el.offsetTop;
+
+			if (this.isBottom) {
+				this.isBottom = false;
+				const overflow = this.el.clientHeight - window.innerHeight;
+				this.spacer.style.marginTop = `${scrollTop - (overflow + this.marginTop)}px`;
+			}
+		}
+
+		this.lastScrollTop = scrollTop <= 0 ? 0 : scrollTop;
+	}
+}