rustdoc: improve scroll locking in the rustdoc mobile sidebars

This commit ports the scroll locking behavior from the source sidebar to the
regular sidebar, and also fixes some bad behavior where opening a "mobile"
sidebar, and growing the viewport so that the "desktop" mode without scroll
locking is activated, could potentially leave the page stuck.

This does not affect the behavior on larger screens. Only small ones, where
the sidebar covers up the main content.

1 files changed, 17 insertions, 3 deletions

diff --git a/src/librustdoc/html/static/js/source-script.js b/src/librustdoc/html/static/js/source-script.js
index 1e9bfa5cc89..c5bfc00c78b 100644
--- a/src/librustdoc/html/static/js/source-script.js
+++ b/src/librustdoc/html/static/js/source-script.js
@@ -10,7 +10,7 @@
 (function() {
 
 const rootPath = document.getElementById("rustdoc-vars").attributes["data-root-path"].value;
-let oldScrollPosition = 0;
+let oldScrollPosition = null;
 
 function closeSidebarIfMobile() {
     if (window.innerWidth < window.RUSTDOC_MOBILE_BREAKPOINT) {
@@ -71,18 +71,21 @@ function toggleSidebar() {
             oldScrollPosition = window.scrollY;
             document.body.style.position = "fixed";
             document.body.style.top = `-${oldScrollPosition}px`;
+        } else {
+            oldScrollPosition = null;
         }
         addClass(document.documentElement, "source-sidebar-expanded");
         child.innerText = "<";
         updateLocalStorage("source-sidebar-show", "true");
     } else {
-        if (window.innerWidth < window.RUSTDOC_MOBILE_BREAKPOINT) {
+        if (window.innerWidth < window.RUSTDOC_MOBILE_BREAKPOINT && oldScrollPosition !== null) {
             // This is to keep the scroll position on mobile.
             document.body.style.position = "";
             document.body.style.top = "";
             // The scroll position is lost when resetting the style, hence why we store it in
-            // `oldScroll`.
+            // `oldScrollPosition`.
             window.scrollTo(0, oldScrollPosition);
+            oldScrollPosition = null;
         }
         removeClass(document.documentElement, "source-sidebar-expanded");
         child.innerText = ">";
@@ -90,6 +93,17 @@ function toggleSidebar() {
     }
 }
 
+window.addEventListener("resize", () => {
+    if (window.innerWidth >= window.RUSTDOC_MOBILE_BREAKPOINT && oldScrollPosition !== null) {
+        // If the user opens the sidebar in "mobile" mode, and then grows the browser window,
+        // we need to switch away from mobile mode and make the main content area scrollable.
+        document.body.style.position = "";
+        document.body.style.top = "";
+        window.scrollTo(0, oldScrollPosition);
+        oldScrollPosition = null;
+    }
+});
+
 function createSidebarToggle() {
     const sidebarToggle = document.createElement("div");
     sidebarToggle.id = "sidebar-toggle";