Fixes firefox copy paste issue

Author: gstjepan2

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

@@ -2138,3 +2138,28 @@ function preLoadCss(cssUrl) {
         elem.addEventListener("click", showHideCodeExampleButtons);
     });
 }());
+
+// This section is a bugfix for firefox: when copying text with `user-select: none`, it adds
+// extra backline characters.
+//
+// Rustdoc issue: Workaround for https://github.com/rust-lang/rust/issues/141464
+// Firefox issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1273836
+(function() {
+    document.body.addEventListener("copy", event => {
+        let target = event.target;
+        let isInsideCode = false;
+        while (target !== document.body) {
+            if (target.tagName === "CODE") {
+                isInsideCode = true;
+                break;
+            }
+            target = target.parentElement;
+        }
+        if (!isInsideCode) {
+            return;
+        }
+        const selection = document.getSelection();
+        nonnull(event.clipboardData).setData("text/plain", selection.toString());
+        event.preventDefault();
+    });
+}());