refactor: simplify auto header insertion logic

docs/configuration.md

@@ -72,7 +72,9 @@ window.$docsify = {
 - Type: `Boolean`
 - Default: `false`
 
-If `loadSidebar` and `autoHeader` are both enabled, for each link in `_sidebar.md`, prepend a header to the page before converting it to HTML. See [#78](https://github.com/docsifyjs/docsify/issues/78).
+If `loadSidebar` and `autoHeader` are both enabled, for each link in `_sidebar.md`, prepend a header to the page before converting it to HTML — but only if the page does not already contain an H1 heading.
+
+For more details, see [#78](https://github.com/docsifyjs/docsify/issues/78).
 
 ```js
 window.$docsify = {

src/core/render/index.js

@@ -342,11 +342,15 @@ export function Render(Base) {
 
       if (autoHeader && activeEl) {
         const main = dom.getNode('#main');
-        const firstNode = main.children[0];
-        if (firstNode && firstNode.tagName !== 'H1') {
-          const h1 = this.compiler.header(activeEl.innerText, 1);
-          const wrapper = dom.create('div', h1);
-          dom.before(main, wrapper.children[0]);
+        const hasH1 = main.querySelector('h1');
+
+        if (!hasH1) {
+          const h1HTML = this.compiler.header(activeEl.innerText, 1);
+          const h1Node = dom.create('div', h1HTML).firstElementChild;
+
+          if (h1Node) {
+            dom.before(main, h1Node);
+          }
         }
       }
     }