rustdoc: use CSS overscroll-behavior instead of JavaScript

Fixes the desktop scrolling weirdness mentioned in
rust-lang#98775 (comment)

As described in the MDN page for this property:

* The current Firefox ESR is 102, and the first Firefox version
  to support this feature is 59.
* The current Chrome version 112, and the first version to support
  this is 63.
* Edge is described as having a minor bug in `none` mode, but we
  use `contain` mode anyway, so it doesn't matter.
* Safari 16, released September 2022, is the last browser to
  add this feature, and is also the oldest version we officially
  support.

src/librustdoc/html/static/css/rustdoc.css

@@ -384,6 +384,7 @@ img {
 	font-size: 0.875rem;
 	flex: 0 0 200px;
 	overflow-y: scroll;
+	overscroll-behavior: contain;
 	position: sticky;
 	height: 100vh;
 	top: 0;
@@ -1531,7 +1532,7 @@ However, it's not needed with smaller screen width because the doc/code block is
 /*
 WARNING: RUSTDOC_MOBILE_BREAKPOINT MEDIA QUERY
 If you update this line, then you also need to update the line with the same warning
-in main.js
+in source-script.js
 */
 @media (max-width: 700px) {
 	/* When linking to an item with an `id` (for instance, by clicking a link in the sidebar,

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

@@ -4,11 +4,6 @@
 
 "use strict";
 
-// WARNING: RUSTDOC_MOBILE_BREAKPOINT MEDIA QUERY
-// If you update this line, then you also need to update the media query with the same
-// warning in rustdoc.css
-window.RUSTDOC_MOBILE_BREAKPOINT = 700;
-
 // Given a basename (e.g. "storage") and an extension (e.g. ".js"), return a URL
 // for a resource under the root-path, with the resource-suffix.
 function resourcePath(basename, extension) {
@@ -730,65 +725,18 @@ function preLoadCss(cssUrl) {
         window.rustdoc_add_line_numbers_to_examples();
     }
 
-    let oldSidebarScrollPosition = null;
-
-    // Scroll locking used both here and in source-script.js
-
-    window.rustdocMobileScrollLock = function() {
-        const mobile_topbar = document.querySelector(".mobile-topbar");
-        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`;
-            if (mobile_topbar) {
-                mobile_topbar.style.top = `${oldSidebarScrollPosition}px`;
-                mobile_topbar.style.position = "relative";
-            }
-        } else {
-            oldSidebarScrollPosition = null;
-        }
-    };
-
-    window.rustdocMobileScrollUnlock = function() {
-        const mobile_topbar = document.querySelector(".mobile-topbar");
-        if (oldSidebarScrollPosition !== null) {
-            // This is to keep the scroll position on mobile.
-            document.body.style.width = "";
-            document.body.style.position = "";
-            document.body.style.top = "";
-            if (mobile_topbar) {
-                mobile_topbar.style.top = "";
-                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;
-        }
-    };
-
     function showSidebar() {
         window.hideAllModals(false);
-        window.rustdocMobileScrollLock();
         const sidebar = document.getElementsByClassName("sidebar")[0];
         addClass(sidebar, "shown");
     }
 
     function hideSidebar() {
-        window.rustdocMobileScrollUnlock();
         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();
-        }
         if (window.CURRENT_TOOLTIP_ELEMENT) {
             // As a workaround to the behavior of `contains: layout` used in doc togglers,
             // tooltip popovers are positioned using javascript.

src/librustdoc/html/static/js/source-script.js

@@ -15,8 +15,13 @@ const NAME_OFFSET = 0;
 const DIRS_OFFSET = 1;
 const FILES_OFFSET = 2;
 
+// WARNING: RUSTDOC_MOBILE_BREAKPOINT MEDIA QUERY
+// If you update this line, then you also need to update the media query with the same
+// warning in rustdoc.css
+const RUSTDOC_MOBILE_BREAKPOINT = 700;
+
 function closeSidebarIfMobile() {
-    if (window.innerWidth < window.RUSTDOC_MOBILE_BREAKPOINT) {
+    if (window.innerWidth < RUSTDOC_MOBILE_BREAKPOINT) {
         updateLocalStorage("source-sidebar-show", "false");
     }
 }
@@ -69,12 +74,10 @@ function createDirEntry(elem, parent, fullPath, hasFoundFile) {
 function toggleSidebar() {
     const child = this.parentNode.children[0];
     if (child.innerText === ">") {
-        window.rustdocMobileScrollLock();
         addClass(document.documentElement, "source-sidebar-expanded");
         child.innerText = "<";
         updateLocalStorage("source-sidebar-show", "true");
     } else {
-        window.rustdocMobileScrollUnlock();
         removeClass(document.documentElement, "source-sidebar-expanded");
         child.innerText = ">";
         updateLocalStorage("source-sidebar-show", "false");

tests/rustdoc-gui/sidebar-mobile-scroll.goml

@@ -1,31 +1,12 @@
-// This test ensures that the mobile sidebar preserves scroll position.
+// This test ensures that the mobile disables scrolling the page.
 go-to: "file://" + |DOC_PATH| + "/test_docs/struct.Foo.html"
-// Switching to "mobile view" by reducing the width to 600px.
-set-window-size: (700, 600)
-assert-css: (".sidebar", {"display": "block", "left": "-1000px"})
-
-// Scroll down.
-scroll-to: "//h2[@id='blanket-implementations']"
-assert-window-property: {"pageYOffset": "622"}
-
-// 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": "622"}
-
-// 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"}
-set-window-size: (900, 600)
-assert-window-property: {"pageYOffset": "622"}
+set-window-size: (1280, 800) // desktop
+assert-css: (".sidebar", {"overscroll-behavior": "contain"})
+set-window-size: (700, 600) // mobile
+assert-css: (".sidebar", {"overscroll-behavior": "contain"})
+
+go-to: "file://" + |DOC_PATH| + "/src/test_docs/lib.rs.html"
+set-window-size: (1280, 800) // desktop
+assert-css: (".sidebar", {"overscroll-behavior": "contain"})
+set-window-size: (700, 600) // mobile
+assert-css: (".sidebar", {"overscroll-behavior": "contain"})

tests/rustdoc-gui/sidebar-source-code-display.goml

@@ -183,22 +183,12 @@ wait-for-css: (".sidebar", {"left": "-1000px"})
 // The "scrollTop" property should be the same.
 assert-window-property: {"pageYOffset": "2542"}
 
-// We now check that the scroll position is restored if the window is resized.
-set-window-size: (500, 700)
-click: "#src-sidebar-toggle"
-wait-for-css: ("#source-sidebar", {"visibility": "visible"})
-assert-window-property: {"pageYOffset": "0"}
-set-window-size: (900, 900)
-assert-window-property: {"pageYOffset": "2542"}
-set-window-size: (500, 700)
-click: "#src-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
 // you click one of them, you probably want to actually see the file's contents, and not just
 // make it the current selection.
+set-window-size: (500, 700)
 click: "#src-sidebar-toggle"
 wait-for-css: ("#source-sidebar", {"visibility": "visible"})
 assert-local-storage: {"rustdoc-source-sidebar-show": "true"}