Fix scroll offset when jumping to internal id

Author: jsha

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

@@ -1738,12 +1738,19 @@ details.rustdoc-toggle[open] > summary.hideme::after {
 }
 
 @media (max-width: 700px) {
+	/* When linking to an item with an `id` (for instance, by clicking a link in the sidebar,
+	   or visiting a URL with a fragment like `#method.new`, we don't want the item to be obscured
+	   by the topbar. Anything with an `id` gets scroll-margin-top equal to .mobile-topbar's size.
+	*/
+	*[id] {
+		scroll-margin-top: 45px;
+	}
+
 	.rustdoc {
 		padding-top: 0px;
 		/* Sidebar should overlay main content, rather than pushing main content to the right.
 		   Turn off `display: flex` on the body element. */
 		display: block;
-		scroll-margin-top: 45px;
 	}
 
 	main {

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

@@ -29,3 +29,9 @@ assert-css: (".sidebar", {"display": "block", "left": "-1000px"})
 
 // Check that the topbar is visible
 assert-property: (".mobile-topbar", {"clientHeight": "45"})
+
+// Check that clicking an element from the sidebar scrolls to the right place
+// so the target is not obscured by the topbar.
+click: ".sidebar-menu-toggle"
+click: ".sidebar-links a"
+assert-position: ("#method\.must_use", {"y": 45})