rustdoc: improve scroll locking in the rustdoc mobile sidebars

src/librustdoc/html/static/js/main.js

@@ -348,8 +348,7 @@ function loadCss(cssFileName) {
 
     function onHashChange(ev) {
         // If we're in mobile mode, we should hide the sidebar in any case.
-        const sidebar = document.getElementsByClassName("sidebar")[0];
-        removeClass(sidebar, "shown");
+        hideSidebar();
         handleHashes(ev);
     }
 
@@ -731,11 +730,50 @@ function loadCss(cssFileName) {
         });
     }());
 
+    let oldSidebarScrollPosition = null;
+
+    function showSidebar() {
+        if (window.innerWidth < window.RUSTDOC_MOBILE_BREAKPOINT) {
+            // This is to keep the scroll position on mobile.
+            oldSidebarScrollPosition = window.scrollY;
+            document.body.style.width = `${document.body.offsetWidth}px`;
+            document.body.style.position = "fixed";
+            document.body.style.top = `-${oldSidebarScrollPosition}px`;
+            document.querySelector(".mobile-topbar").style.top = `${oldSidebarScrollPosition}px`;
+            document.querySelector(".mobile-topbar").style.position = "relative";
+        } else {
+            oldSidebarScrollPosition = null;
+        }
+        const sidebar = document.getElementsByClassName("sidebar")[0];
+        addClass(sidebar, "shown");
+    }
+
     function hideSidebar() {
+        if (oldSidebarScrollPosition !== null) {
+            // This is to keep the scroll position on mobile.
+            document.body.style.width = "";
+            document.body.style.position = "";
+            document.body.style.top = "";
+            document.querySelector(".mobile-topbar").style.top = "";
+            document.querySelector(".mobile-topbar").style.position = "";
+            // The scroll position is lost when resetting the style, hence why we store it in
+            // `oldSidebarScrollPosition`.
+            window.scrollTo(0, oldSidebarScrollPosition);
+            oldSidebarScrollPosition = null;
+        }
         const sidebar = document.getElementsByClassName("sidebar")[0];
         removeClass(sidebar, "shown");
     }
 
+    window.addEventListener("resize", () => {
+        if (window.innerWidth >= window.RUSTDOC_MOBILE_BREAKPOINT &&
+            oldSidebarScrollPosition !== 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.
+            hideSidebar();
+        }
+    });
+
     function handleClick(id, f) {
         const elem = document.getElementById(id);
         if (elem) {
@@ -778,9 +816,9 @@ function loadCss(cssFileName) {
         sidebar_menu_toggle.addEventListener("click", () => {
             const sidebar = document.getElementsByClassName("sidebar")[0];
             if (!hasClass(sidebar, "shown")) {
-                addClass(sidebar, "shown");
+                showSidebar();
             } else {
-                removeClass(sidebar, "shown");
+                hideSidebar();
             }
         });
     }

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) {
@@ -71,25 +71,39 @@ 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 = ">";
         updateLocalStorage("source-sidebar-show", "false");
     }
 }
 
+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";

src/test/rustdoc-gui/sidebar-mobile-scroll.goml

@@ -0,0 +1,31 @@
+// This test ensures that the mobile sidebar preserves scroll position.
+goto: file://|DOC_PATH|/test_docs/struct.Foo.html
+// Switching to "mobile view" by reducing the width to 600px.
+size: (600, 600)
+assert-css: (".sidebar", {"display": "block", "left": "-1000px"})
+
+// Scroll down.
+scroll-to: "//h2[@id='blanket-implementations']"
+assert-window-property: {"pageYOffset": "702"}
+
+// Open the sidebar menu.
+click: ".sidebar-menu-toggle"
+wait-for-css: (".sidebar", {"left": "0px"})
+
+// We are no longer "scrolled". It's important that the user can't
+// scroll the body at all, but these test scripts are run only in Chrome,
+// and we need to use a more complicated solution to this problem because
+// of Mobile Safari...
+assert-window-property: {"pageYOffset": "0"}
+
+// Close the sidebar menu. Make sure the scroll position gets restored.
+click: ".sidebar-menu-toggle"
+wait-for-css: (".sidebar", {"left": "-1000px"})
+assert-window-property: {"pageYOffset": "702"}
+
+// Now test that scrollability returns when the browser window is just resized.
+click: ".sidebar-menu-toggle"
+wait-for-css: (".sidebar", {"left": "0px"})
+assert-window-property: {"pageYOffset": "0"}
+size: (900, 900)
+assert-window-property: {"pageYOffset": "702"}

src/test/rustdoc-gui/sidebar-source-code-display.goml

@@ -233,6 +233,17 @@ wait-for-css: (".sidebar", {"width": "0px"})
 // The "scrollTop" property should be the same.
 assert-window-property: {"pageYOffset": "2519"}
 
+// We now check that the scroll position is restored if the window is resized.
+size: (500, 700)
+click: "#sidebar-toggle"
+wait-for-css: ("#source-sidebar", {"visibility": "visible"})
+assert-window-property: {"pageYOffset": "0"}
+size: (900, 900)
+assert-window-property: {"pageYOffset": "2519"}
+size: (500, 700)
+click: "#sidebar-toggle"
+wait-for-css: ("#source-sidebar", {"visibility": "hidden"})
+
 // We now check that opening the sidebar and clicking a link will close it.
 // The behavior here on mobile is different than the behavior on desktop,
 // but common sense dictates that if you have a list of files that fills the entire screen, and