Fixed js bug on visited function

custom/custom.js

 /*** Autoscroll ***/
 let autoscroll = () => {
-    container = document.querySelector("#site");
-    cell = document.querySelector(".cell.selected");
+	container = document.querySelector("#site");
+	cell = document.querySelector(".cell.selected");
 
-    dy = (container.clientHeight - cell.clientHeight) / 2;
+	dy = (container.clientHeight - cell.clientHeight) / 2;
 
-    if (dy < 0) dy = 0;
+	if (dy < 0) dy = 0;
 
-    container.scrollTop = cell.offsetTop - dy;
+	container.scrollTop = cell.offsetTop - dy;
 };
 
 $([IPython.events]).on("select.Cell", autoscroll);
 $([IPython.events]).on("output_appended.OutputArea", function () {
-    setTimeout(autoscroll, 10);
+	setTimeout(autoscroll, 10);
 });
 
-/*** Auto reveal ***/
+/*** visited cell marker ***/
 (() => {
-    let style = document.createElement('style');
+	let style = document.createElement('style');
 
-    document.head.appendChild(style);
+	document.head.appendChild(style);
 
-    style.type = 'text/css';
-    style.innerText = ".unselected { visibility: hidden;}";
+	style.type = 'text/css';
+	style.innerText = ".cell { visibility: hidden;}";
 
+	setTimeout(() => {
+		document.querySelector(".cell.selected").classList.add("visited");
+		document.querySelectorAll(".cell.unselected").forEach(e => {
+			e.classList.add("unvisited");
+		});
 
-    document.querySelectorAll(".cell:not(:first-of-type)").forEach(e => {
-        e.classList.add("unselected");
-    });
-    
-    $([IPython.events]).on("select.Cell", () => {
-        document.querySelector(".cell.selected").classList.remove("unselected");
-    });
+		style.innerText = ".cell.unvisited { visibility: hidden;}";
+
+		$([IPython.events]).on("select.Cell", () => {
+			document.querySelector(".cell.selected").classList.add("visited");
+			document.querySelector(".cell.selected").classList.remove("unvisited");
+		});
+	}, 200);
 })();
\ No newline at end of file