rustdoc: use .rustdoc class instead of body

This didn't show up in our local tests, because the problem is actually
caused by docs.rs rewritten HTML (which relocates the classes that this
code looked for from the body tag to a child div).

Fixes #117290

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

@@ -54,7 +54,7 @@ function setMobileTopbar() {
     if (mobileTopbar) {
         const mobileTitle = document.createElement("h2");
         mobileTitle.className = "location";
-        if (hasClass(document.body, "crate")) {
+        if (hasClass(document.querySelector(".rustdoc"), "crate")) {
             mobileTitle.innerText = `Crate ${window.currentCrate}`;
         } else if (locationTitle) {
             mobileTitle.innerHTML = locationTitle.innerHTML;
@@ -485,7 +485,7 @@ function preLoadCss(cssUrl) {
                 return;
             }
 
-            const modpath = hasClass(document.body, "mod") ? "../" : "";
+            const modpath = hasClass(document.querySelector(".rustdoc"), "mod") ? "../" : "";
 
             const h3 = document.createElement("h3");
             h3.innerHTML = `<a href="${modpath}index.html#${id}">${longty}</a>`;