diff --git a/.changelog/5971-fix-react-19-follow-ups.md b/.changelog/5971-fix-react-19-follow-ups.md
new file mode 100644
index 0000000000..f81937d90d
--- /dev/null
+++ b/.changelog/5971-fix-react-19-follow-ups.md
@@ -0,0 +1,3 @@
+### Fixed
+
+- Fixed the IIFE build of the Fluent theme crashing on load with React 19, in PR [#5971](https://github.com/microsoft/BotFramework-WebChat/pull/5971), by [@OEvgeny](https://github.com/OEvgeny)
diff --git a/__tests__/html2/react19/codeBlockCopyButton.html b/__tests__/html2/react19/codeBlockCopyButton.html
new file mode 100644
index 0000000000..44caa881a2
--- /dev/null
+++ b/__tests__/html2/react19/codeBlockCopyButton.html
@@ -0,0 +1,92 @@
+<!doctype html>
+<html lang="en-US">
+  <head>
+    <title>React 19: renders the copy button of code blocks</title>
+    <link href="/assets/index.css" rel="stylesheet" type="text/css" />
+    <script type="importmap">
+      {
+        "imports": {
+          "@testduet/wait-for": "https://esm.sh/@testduet/wait-for",
+          "react": "https://esm.sh/react@19.3.1?dev",
+          "react-dom": "https://esm.sh/react-dom@19.3.1?dev",
+          "react-dom/client": "https://esm.sh/react-dom@19.3.1/client?dev"
+        }
+      }
+    </script>
+    <script crossorigin="anonymous" src="/test-harness.js"></script>
+    <script crossorigin="anonymous" src="/test-page-object.js"></script>
+    <script type="module">
+      // React 19 has no UMD build: the IIFE build of Web Chat uses `window.React` and `window.ReactDOM` when they are set.
+      import React from 'react';
+      import ReactDOM from 'react-dom';
+
+      window.React = React;
+      window.ReactDOM = ReactDOM;
+    </script>
+    <script crossorigin="anonymous" defer src="/__dist__/webchat-es5.js"></script>
+  </head>
+  <body>
+    <main id="webchat"></main>
+    <script type="module">
+      import { waitFor } from '@testduet/wait-for';
+      import { createElement, version } from 'react';
+      import ReactDOM from 'react-dom';
+      import { createRoot } from 'react-dom/client';
+
+      run(async function () {
+        await host.sendDevToolsCommand('Browser.setPermission', {
+          permission: { name: 'clipboard-write' },
+          setting: 'granted'
+        });
+
+        const {
+          WebChat: { ReactWebChat, testIds }
+        } = window; // Imports in UMD fashion.
+
+        const { directLine, store } = testHelpers.createDirectLineEmulator();
+
+        // GIVEN: Web Chat rendered with React 19, which no longer has `ReactDOM.render()`.
+        expect(version).toBe('19.3.1');
+        expect(ReactDOM.version).toEqual(expect.stringMatching(/^19\./u));
+        expect(ReactDOM.render).toBeUndefined();
+
+        createRoot(document.getElementById('webchat')).render(
+          createElement(ReactWebChat, { directLine, store, styleOptions: { hideSendBox: true } })
+        );
+
+        await pageConditions.uiConnected();
+
+        // WHEN: The bot sends a code block.
+        await directLine.emulateIncomingActivity({
+          text: 'Here is the code:\n\n```\nconsole.log("Hello, World!");\n```',
+          type: 'message'
+        });
+
+        await pageConditions.numActivitiesShown(1);
+
+        // THEN: The code block should have a "Copy" button.
+        const copyButton = await waitFor(() => {
+          const element = document.querySelector(`[data-testid="${testIds.codeBlockCopyButton}"]`);
+
+          expect(element).toBeTruthy();
+
+          return element;
+        });
+
+        const { name, role } = await testHelpers.getAccessibilityNode(copyButton);
+
+        expect(name.value).toBe('Copy');
+        expect(role.value).toBe('button');
+
+        // WHEN: The "Copy" button is clicked.
+        await host.click(copyButton);
+
+        // THEN: The button should change to "Copied".
+        await waitFor(async () => expect(await testHelpers.getAccessibleName(copyButton)).toBe('Copied'));
+
+        // THEN: The code block shows the copy button with a check mark.
+        await host.snapshot('local');
+      });
+    </script>
+  </body>
+</html>
diff --git a/__tests__/html2/react19/codeBlockCopyButton.html.snap-1.png b/__tests__/html2/react19/codeBlockCopyButton.html.snap-1.png
new file mode 100644
index 0000000000..f931af3deb
Binary files /dev/null and b/__tests__/html2/react19/codeBlockCopyButton.html.snap-1.png differ
diff --git a/__tests__/html2/react19/fluentTheme.iife.html b/__tests__/html2/react19/fluentTheme.iife.html
new file mode 100644
index 0000000000..d94578f86f
--- /dev/null
+++ b/__tests__/html2/react19/fluentTheme.iife.html
@@ -0,0 +1,80 @@
+<!doctype html>
+<html lang="en-US">
+  <head>
+    <title>React 19: the IIFE build of the Fluent theme sends a message and shows the reply</title>
+    <link href="/assets/index.css" rel="stylesheet" type="text/css" />
+    <script type="importmap">
+      {
+        "imports": {
+          "react": "https://esm.sh/react@19.3.1?dev",
+          "react-dom": "https://esm.sh/react-dom@19.3.1?dev",
+          "react-dom/client": "https://esm.sh/react-dom@19.3.1/client?dev"
+        }
+      }
+    </script>
+    <script crossorigin="anonymous" src="/test-harness.js"></script>
+    <script crossorigin="anonymous" src="/test-page-object.js"></script>
+    <script type="module">
+      // React 19 has no UMD build: the IIFE builds of Web Chat and the Fluent theme use `window.React` and `window.ReactDOM`.
+      import React from 'react';
+      import ReactDOM from 'react-dom';
+
+      window.React = React;
+      window.ReactDOM = ReactDOM;
+    </script>
+    <script crossorigin="anonymous" defer src="/__dist__/webchat-es5.js"></script>
+    <script crossorigin="anonymous" defer src="/__dist__/botframework-webchat-fluent-theme.development.js"></script>
+  </head>
+  <body>
+    <main id="webchat"></main>
+    <script type="module">
+      import { createElement, version } from 'react';
+      import ReactDOM from 'react-dom';
+      import { createRoot } from 'react-dom/client';
+
+      run(async function () {
+        const {
+          WebChat: { FluentThemeProvider, ReactWebChat }
+        } = window; // Imports in UMD fashion.
+
+        const { directLine, store } = testHelpers.createDirectLineEmulator();
+
+        // GIVEN: The Fluent theme rendered with React 19, which no longer has `ReactDOM.render()`.
+        expect(version).toBe('19.3.1');
+        expect(ReactDOM.version).toEqual(expect.stringMatching(/^19\./u));
+        expect(ReactDOM.render).toBeUndefined();
+
+        createRoot(document.getElementById('webchat')).render(
+          createElement(FluentThemeProvider, {}, createElement(ReactWebChat, { directLine, store }))
+        );
+
+        await pageConditions.uiConnected();
+
+        // WHEN: A message is sent through the send box.
+        const sendMessage = await directLine.actPostActivity(() =>
+          pageObjects.sendMessageViaSendBox('Aloha!', { waitForSend: false })
+        );
+
+        await sendMessage.resolveAll();
+
+        // WHEN: The bot replies.
+        await directLine.emulateIncomingActivity({
+          from: { id: 'bot', role: 'bot' },
+          text: 'Aloha! How can I help?',
+          type: 'message'
+        });
+
+        // THEN: The transcript shows the message and the reply.
+        await pageConditions.numActivitiesShown(2);
+
+        const [sent, reply] = pageElements.activities();
+
+        expect(sent.textContent).toContain('Aloha!');
+        expect(reply.textContent).toContain('Aloha! How can I help?');
+
+        // THEN: The Fluent theme shows the message, the reply and its send box.
+        await host.snapshot('local');
+      });
+    </script>
+  </body>
+</html>
diff --git a/__tests__/html2/react19/fluentTheme.iife.html.snap-1.png b/__tests__/html2/react19/fluentTheme.iife.html.snap-1.png
new file mode 100644
index 0000000000..ca3f67cbf3
Binary files /dev/null and b/__tests__/html2/react19/fluentTheme.iife.html.snap-1.png differ
diff --git a/__tests__/html2/react19/renderWebChat.esm.html b/__tests__/html2/react19/renderWebChat.esm.html
index ac79ff7a4d..708d95f68c 100644
--- a/__tests__/html2/react19/renderWebChat.esm.html
+++ b/__tests__/html2/react19/renderWebChat.esm.html
@@ -8,7 +8,7 @@
     <main id="webchat"></main>
     <!--
       React 18 with "react-dom" emulating React 19 (no `ReactDOM.render()`).
-      TODO: Use React 19 when https://github.com/spyip/react-film/pull/110 is released and bumped: the static build cannot load with React 19 until then.
+      TODO: Use React 19 once the static build no longer bundles react-film, which imports `render` from "react-dom" (removed, or https://github.com/spyip/react-film/pull/110 released and bumped): it fails to load with React 19 until then.
     -->
     <script type="importmap">
       {
@@ -37,7 +37,8 @@
         const { directLine, store } = testHelpers.createDirectLineEmulator();
         const webChatElement = document.getElementById('webchat');
 
-        // GIVEN: "react-dom" without `ReactDOM.render()`, like React 19.
+        // GIVEN: "react-dom" of React 18 without `ReactDOM.render()`, like React 19.
+        expect(ReactDOM.version).toEqual(expect.stringMatching(/^18\./u)); // React 18 underneath the emulation, see the import map.
         expect(ReactDOM.render).toBeUndefined();
 
         // WHEN: Web Chat is rendered by renderWebChat() into the element.
diff --git a/__tests__/html2/react19/renderWebChat.iife.html b/__tests__/html2/react19/renderWebChat.iife.html
index 221d62ab38..45bda82b3c 100644
--- a/__tests__/html2/react19/renderWebChat.iife.html
+++ b/__tests__/html2/react19/renderWebChat.iife.html
@@ -43,6 +43,7 @@
 
         // GIVEN: React 19, which no longer has `ReactDOM.render()`.
         expect(version).toBe('19.3.1');
+        expect(ReactDOM.version).toEqual(expect.stringMatching(/^19\./u));
         expect(ReactDOM.render).toBeUndefined();
 
         // WHEN: Web Chat is rendered by renderWebChat() into the element.
@@ -54,7 +55,7 @@
         // WHEN: Web Chat is rendered by renderWebChat() with something that is neither an element nor a root.
         // THEN: It throws.
         expect(() => renderWebChat({ directLine, store }, {})).toThrow(
-          'renderWebChat() expects an HTMLElement or a root from createRoot().'
+          'renderWebChat() expects an HTMLElement or a root from ReactDOM.createRoot().'
         );
 
         // WHEN: Web Chat is rendered by renderWebChat() with a root from createRoot() instead of the element, without the send box, as the error says.
diff --git a/__tests__/html2/react19/renderWebChat.root.react18.esm.html b/__tests__/html2/react19/renderWebChat.root.react18.esm.html
index f11837f58d..c7c19cefff 100644
--- a/__tests__/html2/react19/renderWebChat.root.react18.esm.html
+++ b/__tests__/html2/react19/renderWebChat.root.react18.esm.html
@@ -11,6 +11,7 @@
         "imports": {
           "@testduet/wait-for": "https://esm.sh/@testduet/wait-for",
           "botframework-webchat": "/__dist__/packages/bundle/static/botframework-webchat.js",
+          "jest-mock": "https://esm.sh/jest-mock",
           "react": "/__dist__/packages/bundle/static/react.js",
           "react-dom": "/__dist__/packages/bundle/static/react-dom.js",
           "react-dom/client": "/__dist__/packages/bundle/static/react-dom/client.js"
@@ -23,6 +24,7 @@
 
       import { waitFor } from '@testduet/wait-for';
       import { createStoreWithOptions, renderWebChat, testIds } from 'botframework-webchat';
+      import { spyOn } from 'jest-mock';
       import { version } from 'react';
       import ReactDOM from 'react-dom';
       import { createRoot } from 'react-dom/client';
@@ -37,21 +39,15 @@
         expect(version).toMatch(/^18\./u);
         expect(typeof ReactDOM.render).toBe('function');
 
-        // GIVEN: A root from createRoot(), which records what it renders.
+        // GIVEN: A root from createRoot(), with its `render()` spied on.
         const root = createRoot(document.getElementById('webchat'));
-        const renderedChildren = [];
-        const recordingRoot = {
-          render(children) {
-            renderedChildren.push(children);
-            root.render(children);
-          }
-        };
+        const renderSpy = spyOn(root, 'render');
 
         // WHEN: Web Chat is rendered by renderWebChat() with the root instead of the element, without the send box.
-        renderWebChat({ directLine, store, styleOptions: { hideSendBox: true } }, recordingRoot);
+        renderWebChat({ directLine, store, styleOptions: { hideSendBox: true } }, root);
 
         // THEN: The root rendered Web Chat once.
-        expect(renderedChildren).toHaveLength(1);
+        expect(renderSpy).toHaveBeenCalledTimes(1);
 
         await pageConditions.uiConnected();
 
@@ -66,10 +62,10 @@
         const transcript = pageElements.transcript();
 
         // WHEN: renderWebChat() is called again with the same root, with the send box.
-        renderWebChat({ directLine, store, styleOptions: { hideSendBox: false } }, recordingRoot);
+        renderWebChat({ directLine, store, styleOptions: { hideSendBox: false } }, root);
 
         // THEN: The root rendered Web Chat again.
-        expect(renderedChildren).toHaveLength(2);
+        expect(renderSpy).toHaveBeenCalledTimes(2);
 
         // THEN: The send box appears.
         await waitFor(() => expect(pageElements.sendBoxTextBox()).toBeTruthy());
diff --git a/docs/HOSTING.md b/docs/HOSTING.md
index 993f092618..6c533d5054 100644
--- a/docs/HOSTING.md
+++ b/docs/HOSTING.md
@@ -118,8 +118,7 @@ When deploying to locked down environment, clients may not have access to smart
     "imports": {
       "botframework-webchat": "https://cdn.botframework.com/4.19.0/static/botframework-webchat.js",
       "react": "https://cdn.botframework.com/4.19.0/static/react.baseline.js",
-      "react-dom": "https://cdn.botframework.com/4.19.0/static/react-dom.baseline.js",
-      "react-dom/client": "https://cdn.botframework.com/4.19.0/static/react-dom.baseline/client.js"
+      "react-dom": "https://cdn.botframework.com/4.19.0/static/react-dom.baseline.js"
     }
   }
 </script>
diff --git a/packages/bundle/src/renderWebChat.spec.tsx b/packages/bundle/src/renderWebChat.spec.tsx
index 1ca156f321..b569dfcada 100644
--- a/packages/bundle/src/renderWebChat.spec.tsx
+++ b/packages/bundle/src/renderWebChat.spec.tsx
@@ -44,6 +44,18 @@ test('Root should be rendered by root.render() instead of ReactDOM.render()', ()
   expect(render).not.toHaveBeenCalled();
 });
 
+test('Error from root.render() should be thrown synchronously to the host', () => {
+  // React throws this when rendering with a root after `root.unmount()`.
+  const root = {
+    render: jest.fn(() => {
+      throw new Error('Cannot update an unmounted root.');
+    })
+  };
+
+  expect(() => renderWebChat(ReactWebChat, {}, root)).toThrow('Cannot update an unmounted root.');
+  expect(root.render).toHaveBeenCalledTimes(1);
+});
+
 test('Element on React 19 should throw an error asking for a root', () => {
   // React 19 removed `ReactDOM.render()`.
   Object.assign(ReactDOM, { render: undefined });
@@ -55,5 +67,5 @@ test('Element on React 19 should throw an error asking for a root', () => {
 
 test('Neither a container nor a root should throw', () =>
   expect(() => renderWebChat(ReactWebChat, {}, {} as unknown as HTMLElement)).toThrow(
-    'renderWebChat() expects an HTMLElement or a root from createRoot().'
+    'renderWebChat() expects an HTMLElement or a root from ReactDOM.createRoot().'
   ));
diff --git a/packages/bundle/src/renderWebChat.tsx b/packages/bundle/src/renderWebChat.tsx
index d1e9230573..9f12077a9d 100644
--- a/packages/bundle/src/renderWebChat.tsx
+++ b/packages/bundle/src/renderWebChat.tsx
@@ -1,6 +1,7 @@
+import { reactNode } from '@msinternal/botframework-webchat-react-valibot';
 import React, { type ComponentType, type ReactNode } from 'react';
 import ReactDOM from 'react-dom';
-import { custom, function_, is, object, picklist } from 'valibot';
+import { args, custom, function_, is, object, picklist, pipe, tuple } from 'valibot';
 
 /** A root from `createRoot()` of "react-dom/client". */
 type RenderWebChatRoot = { render(children: ReactNode): void };
@@ -26,13 +27,16 @@ const DOCUMENT_NODE = 9;
 const DOCUMENT_FRAGMENT_NODE = 11;
 
 // Containers `ReactDOM.render()` accepts: element, document and document fragment, by `nodeType` to work across realms.
-// Checked before `renderSchema`: some custom elements (e.g. Lit) have their own `render()` method.
+// Checked before `reactRootSchema`: some custom elements (e.g. Lit) have their own `render()` method.
 const containerSchema = custom<Exclude<RenderWebChatContainer, null>>(value =>
   is(object({ nodeType: picklist([DOCUMENT_FRAGMENT_NODE, DOCUMENT_NODE, ELEMENT_NODE]) }), value)
 );
 
-// Both `ReactDOM.render()` (React 18 or earlier) and `root.render()`.
-const renderSchema = object({ render: function_() });
+// `ReactDOM.render(children, container)`, React 18 or earlier.
+const legacyReactDOMSchema = object({ render: pipe(function_(), args(tuple([reactNode(), containerSchema]))) });
+
+// `root.render(children)`, root from `createRoot()` of "react-dom/client".
+const reactRootSchema = object({ render: pipe(function_(), args(tuple([reactNode()]))) });
 
 export default function renderWebChat(
   ReactWebChat: ComponentType<any>,
@@ -43,7 +47,7 @@ export default function renderWebChat(
 
   if (is(containerSchema, elementOrRoot)) {
     // React 19 or later removed `ReactDOM.render()`: Web Chat does not import "react-dom/client", the host creates the root.
-    if (!is(renderSchema, ReactDOM)) {
+    if (!is(legacyReactDOMSchema, ReactDOM)) {
       throw new Error(
         'botframework-webchat: renderWebChat() on React 19 or later needs a root: pass createRoot(element) from "react-dom/client" instead of the element.'
       );
@@ -51,11 +55,13 @@ export default function renderWebChat(
 
     // React 16 to 18, and Preact. Also the IIFE build, which bundles React DOM 16.8.6 unless `window.ReactDOM` is set.
     ReactDOM.render(children, elementOrRoot);
-  } else if (is(renderSchema, elementOrRoot)) {
+  } else if (is(reactRootSchema, elementOrRoot)) {
     // The host created the root: render with it on any React version, it is the host's to unmount.
     elementOrRoot.render(children);
   } else {
-    throw new Error('botframework-webchat: renderWebChat() expects an HTMLElement or a root from createRoot().');
+    throw new Error(
+      'botframework-webchat: renderWebChat() expects an HTMLElement or a root from ReactDOM.createRoot().'
+    );
   }
 }
 
diff --git a/packages/fluent-theme/src/external.umd/react-dom.ts b/packages/fluent-theme/src/external.umd/react-dom.ts
new file mode 100644
index 0000000000..ea199e65a0
--- /dev/null
+++ b/packages/fluent-theme/src/external.umd/react-dom.ts
@@ -0,0 +1,4 @@
+/// <reference types="node" />
+
+// The React DOM of the page, as with "react": a bundled React DOM cannot initialize with another React, such as React 19.
+module.exports = (globalThis as any).ReactDOM;
diff --git a/packages/fluent-theme/tsup.config.ts b/packages/fluent-theme/tsup.config.ts
index 291ddd36e2..d665a89a7b 100644
--- a/packages/fluent-theme/tsup.config.ts
+++ b/packages/fluent-theme/tsup.config.ts
@@ -16,6 +16,12 @@ const umdResolvePlugin = {
       path: join(fileURLToPath(import.meta.url), '../src/external.umd/react.ts')
     }));
 
+    // ESBuild use Go regular expressions and does not understand Unicode flag.
+    // eslint-disable-next-line require-unicode-regexp
+    build.onResolve({ filter: /^react-dom$/ }, () => ({
+      path: join(fileURLToPath(import.meta.url), '../src/external.umd/react-dom.ts')
+    }));
+
     // ESBuild use Go regular expressions and does not understand Unicode flag.
     // eslint-disable-next-line require-unicode-regexp
     build.onResolve({ filter: /^botframework-webchat-api$/ }, () => ({