Teact: Fix empty fragment mount position (#5918)

src/components/left/main/AccountMenuItems.tsx

@@ -69,7 +69,7 @@ const AccountMenuItems = ({
   }, [accounts, currentCount, totalLimit]);
 
   return (
-    <div>
+    <>
       {Object.entries(accounts || {})
         .sort(([, account]) => (account.userId === currentUser.id ? -1 : 1))
         .map(([slot, account], index, arr) => {
@@ -116,7 +116,7 @@ const AccountMenuItems = ({
           {lang('MenuAddAccount')}
         </MenuItem>
       )}
-    </div>
+    </>
   );
 };
 

src/components/test/TestFragment2.tsx

@@ -11,12 +11,7 @@ export function App() {
       }}
     >
       <h2>Click to update</h2>
-      {true && (
-        <>
-          {trigger && <span>fragment</span>}
-          {trigger && <span>content</span>}
-        </>
-      )}
+      <FragmentContainer items={trigger ? [1, 2, 3] : []} />
       <div>
         This should always go last.
       </div>
@@ -24,4 +19,12 @@ export function App() {
   );
 }
 
+function FragmentContainer({ items }: { items: number[] }) {
+  return (
+    <>
+      {items.map((n) => <div>{n}</div>)}
+    </>
+  );
+}
+
 export default App;

src/lib/teact/teact-dom.ts

@@ -303,6 +303,15 @@ function mountChildren(
   },
 ) {
   const { children } = $element;
+
+  // Add a placeholder comment node for empty fragments to maintain position
+  if ($element.type === VirtualType.Fragment && children.length === 0) {
+    const fragmentEl = $element as VirtualElementFragment;
+    fragmentEl.placeholderTarget = document.createComment('empty-fragment');
+    insertBefore(options.fragment || parentEl, fragmentEl.placeholderTarget, options.nextSibling);
+    return;
+  }
+
   for (let i = 0, l = children.length; i < l; i++) {
     const $child = children[i];
     const $renderedChild = renderWithVirtual(parentEl, undefined, $child, $element, currentContext, i, options);
@@ -390,7 +399,14 @@ function remount(
 function unmountRealTree($element: VirtualElement) {
   if ($element.type === VirtualType.Component) {
     unmountComponent($element.componentInstance);
-  } else if ($element.type !== VirtualType.Fragment) {
+  } else if ($element.type === VirtualType.Fragment) {
+    // Remove placeholder for empty fragments
+    const fragment = $element as VirtualElementFragment;
+    if (fragment.placeholderTarget && fragment.children.length === 0) {
+      fragment.placeholderTarget.parentNode?.removeChild(fragment.placeholderTarget);
+      fragment.placeholderTarget = undefined;
+    }
+  } else {
     if ($element.type === VirtualType.Tag) {
       extraClasses.delete($element.target!);
       setElementRef($element, undefined);
@@ -419,6 +435,15 @@ function insertBefore(parentEl: DOMElement | DocumentFragment, node: Node, nextS
 
 function getNextSibling($current: VirtualElement): ChildNode | undefined {
   if ($current.type === VirtualType.Component || $current.type === VirtualType.Fragment) {
+    if ($current.children.length === 0) {
+      // For empty fragments, use the placeholder node to track position
+      const fragment = $current as VirtualElementFragment;
+      if (fragment.placeholderTarget) {
+        return fragment.placeholderTarget.nextSibling || undefined;
+      }
+      return undefined;
+    }
+
     const lastChild = $current.children[$current.children.length - 1];
     return getNextSibling(lastChild);
   }
@@ -444,6 +469,28 @@ function renderChildren(
     return;
   }
 
+  // Handle transitions between empty and non-empty fragments
+  if ($current.type === VirtualType.Fragment && $new.type === VirtualType.Fragment) {
+    const currentFragment = $current as VirtualElementFragment;
+    const newFragment = $new as VirtualElementFragment;
+
+    // If transitioning from empty to non-empty, use the placeholder's position
+    if (currentFragment.children.length === 0 && newFragment.children.length > 0 && currentFragment.placeholderTarget) {
+      nextSibling = currentFragment.placeholderTarget.nextSibling || undefined;
+      // Remove the placeholder as we're adding real content
+      currentFragment.placeholderTarget.parentNode?.removeChild(currentFragment.placeholderTarget);
+      currentFragment.placeholderTarget = undefined;
+    }
+
+    // If transitioning from non-empty to empty, add a placeholder
+    if (currentFragment.children.length > 0 && newFragment.children.length === 0) {
+      const lastCurrentChild = currentFragment.children[currentFragment.children.length - 1];
+      const siblingAfterFragment = getNextSibling(lastCurrentChild);
+      newFragment.placeholderTarget = document.createComment('empty-fragment');
+      insertBefore(currentEl, newFragment.placeholderTarget, siblingAfterFragment);
+    }
+  }
+
   const currentChildren = $current.children;
   const newChildren = $new.children;
 

src/lib/teact/teact.ts

@@ -56,6 +56,7 @@ export interface VirtualElementComponent {
 export interface VirtualElementFragment {
   type: VirtualType.Fragment;
   children: VirtualElementChildren;
+  placeholderTarget?: Comment;
 }
 
 export type StateHookSetter<T> = (newValue: ((current: T) => T) | T) => void;