rendered_markdown: Fix code getting copied when visiting playground.

Fixes #29844.

web/src/rendered_markdown.ts

@@ -307,14 +307,15 @@ export const update_elements = ($content: JQuery): void => {
             $view_in_playground_button.attr("data-tippy-content", title);
             $view_in_playground_button.attr("aria-label", title);
         }
-
-        const clipboard = new ClipboardJS($buttonContainer[0], {
+        const $copy_button = $buttonContainer.find(".copy_codeblock");
+        const clipboard = new ClipboardJS($copy_button[0], {
             text(copy_element) {
-                return $(copy_element).siblings("code").text();
+                const $code = $(copy_element).parent().siblings("code");
+                return $code.text();
             },
         });
+
         clipboard.on("success", () => {
-            const $copy_button = $buttonContainer.find(".copy_codeblock");
             show_copied_confirmation($copy_button[0]);
         });
         $codehilite.addClass("zulip-code-block");

web/tests/rendered_markdown.test.js

@@ -529,12 +529,14 @@ function assert_clipboard_setup() {
     assert.equal(clipboard_args[0], "copy-code-stub");
     const text = clipboard_args[1].text({
         to_$: () => ({
-            siblings(arg) {
-                assert.equal(arg, "code");
-                return {
-                    text: () => "text",
-                };
-            },
+            parent: () => ({
+                siblings(arg) {
+                    assert.equal(arg, "code");
+                    return {
+                        text: () => "text",
+                    };
+                },
+            }),
         }),
     });
     assert.equal(text, "text");