Auto merge of #117864 - notriddle:notriddle/rustdoc-root, r=GuillaumeGomez

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

r? `@GuillaumeGomez`

Both problems are regressions introduced by #115948

Author: bors

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>`;