Teact: Fix textContent optimization

src/components/test/TestSetContent.tsx

@@ -0,0 +1,24 @@
+import type { FC } from '../../lib/teact/teact';
+import React, { useState } from '../../lib/teact/teact';
+
+// 1. Make sure "First line" is rendered even if followed by a component with single text.
+// 2. Make sure it then can be normally removed (target is preserved).
+// 3. Make sure "Last line" is also rendered.
+
+const Text: FC = () => {
+  return 'text component';
+};
+
+export function App() {
+  const [withFirstLine, setWithFirstLine] = useState(true);
+
+  return (
+    <div onClick={() => setWithFirstLine((current) => !current)}>
+      {withFirstLine && 'First line'}
+      <Text />
+      Last line
+    </div>
+  );
+}
+
+export default App;

src/lib/teact/teact-dom.ts

@@ -129,9 +129,12 @@ function renderWithVirtual<T extends VirtualElement | undefined>(
 
       mountChildren(parentEl, $new as VirtualElementComponent | VirtualElementFragment, { nextSibling, fragment });
     } else {
-      const canSetTextContent = (
+      const canSetTextContent = !fragment
-        !fragment && !nextSibling && $newAsReal.type === VirtualType.Text && $parent.children.length === 1
+        && !nextSibling
-      );
+        && $newAsReal.type === VirtualType.Text
+        && $parent.children.length === 1
+        && !parentEl.firstChild;
+
       if (canSetTextContent) {
         parentEl.textContent = $newAsReal.value;
         $newAsReal.target = parentEl.firstChild!;