diff --git a/.eslintrc.js b/.eslintrc.js
index 18a3112e7382b..4f902576ad82c 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -577,6 +577,7 @@ module.exports = {
     $AsyncIterator: 'readonly',
     Iterator: 'readonly',
     AsyncIterator: 'readonly',
+    IntervalID: 'readonly',
     IteratorResult: 'readonly',
     JSONValue: 'readonly',
     JSResourceReference: 'readonly',
diff --git a/packages/react-devtools-extensions/src/background/dynamicallyInjectContentScripts.js b/packages/react-devtools-extensions/src/background/dynamicallyInjectContentScripts.js
index f1a3598a519ca..3e2f6b7c1ea83 100644
--- a/packages/react-devtools-extensions/src/background/dynamicallyInjectContentScripts.js
+++ b/packages/react-devtools-extensions/src/background/dynamicallyInjectContentScripts.js
@@ -6,7 +6,7 @@ const contentScriptsToInject = [
     js: ['build/proxy.js'],
     matches: ['<all_urls>'],
     persistAcrossSessions: true,
-    runAt: 'document_end',
+    runAt: 'document_start',
     world: chrome.scripting.ExecutionWorld.ISOLATED,
   },
   {
diff --git a/packages/react-devtools-extensions/src/background/executeScript.js b/packages/react-devtools-extensions/src/background/executeScript.js
index 8b80095d33c2e..a196a7391cdff 100644
--- a/packages/react-devtools-extensions/src/background/executeScript.js
+++ b/packages/react-devtools-extensions/src/background/executeScript.js
@@ -1,6 +1,20 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
 /* global chrome */
 
-export function executeScriptInIsolatedWorld({target, files}) {
+export function executeScriptInIsolatedWorld({
+  target,
+  files,
+}: {
+  files: any,
+  target: any,
+}): Promise<void> {
   return chrome.scripting.executeScript({
     target,
     files,
@@ -8,10 +22,20 @@ export function executeScriptInIsolatedWorld({target, files}) {
   });
 }
 
-export function executeScriptInMainWorld({target, files}) {
+export function executeScriptInMainWorld({
+  target,
+  files,
+  injectImmediately,
+}: {
+  files: any,
+  target: any,
+  // It's nice to have this required to make active choices.
+  injectImmediately: boolean,
+}): Promise<void> {
   return chrome.scripting.executeScript({
     target,
     files,
+    injectImmediately,
     world: chrome.scripting.ExecutionWorld.MAIN,
   });
 }
diff --git a/packages/react-devtools-extensions/src/background/messageHandlers.js b/packages/react-devtools-extensions/src/background/messageHandlers.js
index cd07f8afbc5df..0152418633fbb 100644
--- a/packages/react-devtools-extensions/src/background/messageHandlers.js
+++ b/packages/react-devtools-extensions/src/background/messageHandlers.js
@@ -1,5 +1,6 @@
 /* global chrome */
 
+import {__DEBUG__} from 'react-devtools-shared/src/constants';
 import setExtensionIconAndPopup from './setExtensionIconAndPopup';
 import {executeScriptInMainWorld} from './executeScript';
 
@@ -25,6 +26,7 @@ export function handleBackendManagerMessage(message, sender) {
       payload.versions.forEach(version => {
         if (EXTENSION_CONTAINED_VERSIONS.includes(version)) {
           executeScriptInMainWorld({
+            injectImmediately: true,
             target: {tabId: sender.tab.id},
             files: [`/build/react_devtools_backend_${version}.js`],
           });
@@ -79,9 +81,19 @@ export function handleDevToolsPageMessage(message) {
       }
 
       executeScriptInMainWorld({
+        injectImmediately: true,
         target: {tabId},
         files: ['/build/backendManager.js'],
-      });
+      }).then(
+        () => {
+          if (__DEBUG__) {
+            console.log('Successfully injected backend manager');
+          }
+        },
+        reason => {
+          console.error('Failed to inject backend manager:', reason);
+        },
+      );
 
       break;
     }
diff --git a/packages/react-devtools-extensions/src/contentScripts/proxy.js b/packages/react-devtools-extensions/src/contentScripts/proxy.js
index 8ffeeffb2af53..02253f65d4a83 100644
--- a/packages/react-devtools-extensions/src/contentScripts/proxy.js
+++ b/packages/react-devtools-extensions/src/contentScripts/proxy.js
@@ -1,8 +1,16 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
 /* global chrome */
 
 'use strict';
 
-window.addEventListener('pageshow', function ({target}) {
+function injectProxy({target}: {target: any}) {
   // Firefox's behaviour for injecting this content script can be unpredictable
   // While navigating the history, some content scripts might not be re-injected and still be alive
   if (!window.__REACT_DEVTOOLS_PROXY_INJECTED__) {
@@ -14,7 +22,7 @@ window.addEventListener('pageshow', function ({target}) {
     // The backend waits to install the global hook until notified by the content script.
     // In the event of a page reload, the content script might be loaded before the backend manager is injected.
     // Because of this we need to poll the backend manager until it has been initialized.
-    const intervalID = setInterval(() => {
+    const intervalID: IntervalID = setInterval(() => {
       if (backendInitialized) {
         clearInterval(intervalID);
       } else {
@@ -22,7 +30,11 @@ window.addEventListener('pageshow', function ({target}) {
       }
     }, 500);
   }
-});
+}
+
+window.addEventListener('pagereveal', injectProxy);
+// For backwards compat with browsers not implementing `pagereveal` which is a fairly new event.
+window.addEventListener('pageshow', injectProxy);
 
 window.addEventListener('pagehide', function ({target}) {
   if (target !== window.document) {
@@ -45,7 +57,7 @@ function sayHelloToBackendManager() {
   );
 }
 
-function handleMessageFromDevtools(message) {
+function handleMessageFromDevtools(message: any) {
   window.postMessage(
     {
       source: 'react-devtools-content-script',
@@ -55,7 +67,7 @@ function handleMessageFromDevtools(message) {
   );
 }
 
-function handleMessageFromPage(event) {
+function handleMessageFromPage(event: any) {
   if (event.source !== window || !event.data) {
     return;
   }
@@ -65,6 +77,7 @@ function handleMessageFromPage(event) {
     case 'react-devtools-bridge': {
       backendInitialized = true;
 
+      // $FlowFixMe[incompatible-use]
       port.postMessage(event.data.payload);
       break;
     }
@@ -99,6 +112,8 @@ function connectPort() {
 
   window.addEventListener('message', handleMessageFromPage);
 
+  // $FlowFixMe[incompatible-use]
   port.onMessage.addListener(handleMessageFromDevtools);
+  // $FlowFixMe[incompatible-use]
   port.onDisconnect.addListener(handleDisconnect);
 }
diff --git a/packages/react-devtools-shared/src/__tests__/store-test.js b/packages/react-devtools-shared/src/__tests__/store-test.js
index 2d092ceb0ff06..d4c3fcd9cad4b 100644
--- a/packages/react-devtools-shared/src/__tests__/store-test.js
+++ b/packages/react-devtools-shared/src/__tests__/store-test.js
@@ -987,6 +987,34 @@ describe('Store', () => {
             <Suspense name="two" rects={[{x:1,y:2,width:5,height:1}]}>
             <Suspense name="three" rects={[{x:1,y:2,width:5,height:1}]}>
       `);
+
+      await actAsync(() => {
+        agent.overrideSuspenseMilestone({
+          rendererID,
+          rootID,
+          suspendedSet: [],
+        });
+      });
+
+      expect(store).toMatchInlineSnapshot(`
+        [root]
+          ▾ <App>
+              <Component key="Outside">
+            ▾ <Suspense name="parent">
+                <Component key="Unrelated at Start">
+              ▾ <Suspense name="one">
+                  <Component key="Suspense 1 Content">
+              ▾ <Suspense name="two">
+                  <Component key="Suspense 2 Content">
+              ▾ <Suspense name="three">
+                  <Component key="Suspense 3 Content">
+                <Component key="Unrelated at End">
+        [shell]
+          <Suspense name="parent" rects={[{x:1,y:2,width:5,height:1}, {x:1,y:2,width:5,height:1}, {x:1,y:2,width:5,height:1}, {x:1,y:2,width:5,height:1}, {x:1,y:2,width:5,height:1}]}>
+            <Suspense name="one" rects={[{x:1,y:2,width:5,height:1}]}>
+            <Suspense name="two" rects={[{x:1,y:2,width:5,height:1}]}>
+            <Suspense name="three" rects={[{x:1,y:2,width:5,height:1}]}>
+      `);
     });
 
     it('should display a partially rendered SuspenseList', async () => {
diff --git a/packages/react-devtools-shared/src/backend/fiber/renderer.js b/packages/react-devtools-shared/src/backend/fiber/renderer.js
index 7fd7000d440cb..545725526d04a 100644
--- a/packages/react-devtools-shared/src/backend/fiber/renderer.js
+++ b/packages/react-devtools-shared/src/backend/fiber/renderer.js
@@ -7519,6 +7519,9 @@ export function attach(
     }
 
     // TODO: Allow overriding the timeline for the specified root.
+    forceFallbackForFibers.forEach(fiber => {
+      scheduleUpdate(fiber);
+    });
     forceFallbackForFibers.clear();
 
     for (let i = 0; i < suspendedSet.length; ++i) {
diff --git a/packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseTimeline.js b/packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseTimeline.js
index 04916f9a3646a..59666f624edb7 100644
--- a/packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseTimeline.js
+++ b/packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseTimeline.js
@@ -126,26 +126,28 @@ function SuspenseTimelineInput({rootID}: {rootID: Element['id'] | void}) {
   }
 
   function handleChange(event: SyntheticEvent) {
-    const pendingValue = +event.currentTarget.value;
-    for (let i = 0; i < timeline.length; i++) {
-      const forceFallback = i > pendingValue;
-      const suspense = timeline[i];
-      const elementID = suspense.id;
-      const rendererID = store.getRendererIDForElement(elementID);
-      if (rendererID === null) {
-        // TODO: Handle disconnected elements.
-        console.warn(
-          `No renderer ID found for element ${elementID} in suspense timeline.`,
-        );
-      } else {
-        bridge.send('overrideSuspense', {
-          id: elementID,
-          rendererID,
-          forceFallback,
-        });
-      }
+    if (rootID === undefined) {
+      return;
+    }
+    const rendererID = store.getRendererIDForElement(rootID);
+    if (rendererID === null) {
+      console.error(
+        `No renderer ID found for root element ${rootID} in suspense timeline.`,
+      );
+      return;
     }
 
+    const pendingValue = +event.currentTarget.value;
+    const suspendedSet = timeline
+      .slice(pendingValue + 1)
+      .map(suspense => suspense.id);
+
+    bridge.send('overrideSuspenseMilestone', {
+      rendererID,
+      rootID,
+      suspendedSet,
+    });
+
     const suspense = timeline[pendingValue];
     const elementID = suspense.id;
     highlightHostInstance(elementID);