lazy sessions, restore if visible, forward user after restore

qutebrowser/html/back.html

@@ -1,16 +1,53 @@
 {% extends "base.html" %}
 
 {% block script %}
-setTimeout(function() {
+const STATE_BACK = "back";
-	/* drop first focus event, to avoid problems
+const STATE_FORWARD = "forward";
-	(allow to go easily to newer history entries) */
+
-	window.onfocus = function() {
+function switch_state(new_state) {
-		window.onfocus = null;
+	history.replaceState(
-		window.history.back();
+		new_state,
-	};
+		document.title,
-}, 1000);
+		location.pathname+location.hash);
+}
+
+function go_back() {
+	switch_state(STATE_FORWARD);
+	history.back();
+}
+function go_forward() {
+	switch_state(STATE_BACK);
+	history.forward();
+}
+
+// there are three states
+// default: register focus listener,
+//	    on focus: go back and switch to the state forward
+// back: user came from a later history entry
+//	 -> switch to the state forward,
+//	    forward him to the previous history entry
+// forward: user came from a previous history entry
+//	 -> switch to the state back,
+//	    forward him to the next history entry
+switch (history.state) {
+	case STATE_BACK:
+		go_back();
+		break;
+	case STATE_FORWARD:
+		go_forward();
+		break;
+	default:
+		if (!document.hidden) {
+			go_back();
+			break;
+		}
+		
+		document.addEventListener("visibilitychange", go_back);
+		break;
+}
 {% endblock %}
 
 {% block content %}
 <noscript><p>Javascript isn't enabled. So you need to manually go back in history to restore this tab.</p></noscript>
+<p>If you see this site something went wrong or you reached the end of the history or you disabled javascript.</p>
 {% endblock %}