From 0acec3921413734dae213084d5b7b550e84b40e1 Mon Sep 17 00:00:00 2001
From: Kohei Yoshino <kohei.yoshino@gmail.com>
Date: Thu, 18 Jan 2018 00:19:06 -0500
Subject: Bug 1429785 - In-page back navigation broken

---
 js/global.js | 63 ++++++++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 46 insertions(+), 17 deletions(-)

(limited to 'js')

diff --git a/js/global.js b/js/global.js
index 68aceb03f..10593fb3b 100644
--- a/js/global.js
+++ b/js/global.js
@@ -191,14 +191,48 @@ $().ready(function() {
     });
 });
 
+/**
+ * Emulate the `scrollend` event on an HTML element that will be fired once a
+ * certain amount of scroll is complete.
+ * @param {Element} $target - An element to be observed.
+ */
+const emulate_scrollend_event = $target => {
+    if ('onscrollend' in $target) {
+        return;
+    }
+
+    let timer;
+
+    $target.addEventListener('scroll', () => {
+        window.requestAnimationFrame(() => {
+            window.clearTimeout(timer);
+            timer = window.setTimeout(() => $target.dispatchEvent(new UIEvent('scrollend')), 100);
+        });
+    });
+
+    $target.onscrollend = null;
+}
+
 /**
  * Focus the main content when the page is loaded and there is no autofocus
  * element, so the user can immediately scroll down the page using keyboard.
+ * Also, remember the scroll position on `<main>` so it can be restored when
+ * `location.hash` is changed.
+ * @see adjust_scroll_onload
  */
 const focus_main_content = () => {
+    const $main = document.querySelector('main');
+
     if (!document.querySelector('[autofocus]')) {
-        document.querySelector('main').focus();
+        $main.focus();
     }
+
+    emulate_scrollend_event($main);
+
+    $main.addEventListener('scrollend', () => {
+        history.replaceState(Object.assign(history.state || {}, { main_scroll_top: $main.scrollTop }),
+            document.title, location.href);
+    });
 }
 
 /**
@@ -215,7 +249,9 @@ const detect_blocked_gravatars = () => {
 
 /**
  * If the current URL contains a hash like `#c10`, adjust the scroll position to
- * make some room above the focused element.
+ * make some room above the focused element. Or, restore the scroll position on
+ * `<main>` if remembered.
+ * @see focus_main_content
  */
 const adjust_scroll_onload = () => {
     if (location.hash) {
@@ -224,6 +260,12 @@ const adjust_scroll_onload = () => {
         if ($target) {
             window.setTimeout(() => scroll_element_into_view($target), 50);
         }
+    } else if (history.state) {
+        const main_scroll_top = history.state.main_scroll_top;
+
+        if (typeof main_scroll_top === "number") {
+            document.querySelector('main').scrollTop = main_scroll_top;
+        }
     }
 }
 
@@ -251,21 +293,8 @@ const scroll_element_into_view = ($target, complete) => {
     }
 
     if (typeof complete === 'function') {
-        const callback = () => {
-            $element.removeEventListener('scroll', listener);
-            complete();
-        };
-
-        // Emulate the `scrollend` event
-        const listener = () => {
-            window.clearTimeout(timer);
-            timer = window.setTimeout(callback, 100);
-        };
-
-        // Make sure the callback is always fired even if no scroll happened
-        let timer = window.setTimeout(callback, 100);
-
-        $element.addEventListener('scroll', listener);
+        emulate_scrollend_event($element);
+        $element.addEventListener('scrollend', () => complete(), true);
     }
 
     $element.scrollTop = top - 20;
-- 
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