From 70b83463b084fc2baa19067ec64fcc4ae15cb98c Mon Sep 17 00:00:00 2001
From: Cursor Agent <cursoragent@cursor.com>
Date: Tue, 24 Feb 2026 21:59:17 +0000
Subject: [PATCH 1/5] Add React 19-compatible render profiling fallback

Co-authored-by: Kent C. Dodds <me+github@kentcdodds.com>
---
 .../01.problem.memo/tests/memoized.test.ts    | 110 ++++++++++++++--
 .../01.solution.memo/tests/memoized.test.ts   | 110 ++++++++++++++--
 .../tests/memoized.test.ts                    | 110 ++++++++++++++--
 .../tests/memoized.test.ts                    | 110 ++++++++++++++--
 .../tests/memoized.test.ts                    | 110 ++++++++++++++--
 .../tests/memoized.test.ts                    | 110 ++++++++++++++--
 shared/get-component-calls.tsx                | 121 ++++++++++++++++--
 7 files changed, 691 insertions(+), 90 deletions(-)

diff --git a/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts b/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
index 8f5ee8f83..67806a246 100644
--- a/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
+++ b/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
@@ -12,9 +12,72 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 		const scriptToInject = `
 			<script>
 				let internals
+				const PerformedWorkFlag = 0b000000000000000000000000001
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+
+				let activeFallbackComponentNames = null
+
+				function isUserCodeFiberTag(tag) {
+					return UserCodeFiberTags.has(tag)
+				}
+
+				function getDisplayNameFromType(type) {
+					if (!type) return null
+
+					if (typeof type === 'function') {
+						return type.displayName || type.name || null
+					}
+
+					if (typeof type === 'object') {
+						if (typeof type.displayName === 'string' && type.displayName) {
+							return type.displayName
+						}
+
+						if (typeof type.render === 'function') {
+							return type.render.displayName || type.render.name || null
+						}
+
+						if (type.type) {
+							return getDisplayNameFromType(type.type)
+						}
+					}
+
+					return null
+				}
+
+				function getFiberDisplayName(fiber) {
+					return getDisplayNameFromType(fiber.type) || getDisplayNameFromType(fiber.elementType)
+				}
+
+				function didFiberRender(previousFiber, nextFiber) {
+					if (!previousFiber) return true
+					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
+				}
+
+				function collectRenderedComponentsFromCommit(root) {
+					if (!activeFallbackComponentNames || !root?.current) return
+
+					const stack = [root.current]
+					while (stack.length > 0) {
+						const fiber = stack.pop()
+						if (!fiber) continue
+
+						if (fiber.sibling) stack.push(fiber.sibling)
+						if (fiber.child) stack.push(fiber.child)
+
+						if (!isUserCodeFiberTag(fiber.tag)) continue
+						if (!didFiberRender(fiber.alternate, fiber)) continue
+
+						const componentName = getFiberDisplayName(fiber)
+						if (componentName) {
+							activeFallbackComponentNames.push(componentName)
+						}
+					}
+				}
 
 				function enhanceExistingHook(existingHook) {
 					const originalInject = existingHook.inject
+					const originalCommitFiberRoot = existingHook.onCommitFiberRoot
 
 					existingHook.inject = (injectedInternals) => {
 						internals = injectedInternals
@@ -23,6 +86,12 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 						return originalInject?.call(existingHook, injectedInternals) ?? 1
 					}
 
+					existingHook.onCommitFiberRoot = (...args) => {
+						const [, root] = args
+						collectRenderedComponentsFromCommit(root)
+						return originalCommitFiberRoot?.apply(existingHook, args)
+					}
+
 					return existingHook
 				}
 
@@ -34,7 +103,9 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 							internals = injectedInternals
 							return 1 // Returning a number as React expects a renderer ID
 						},
-						onCommitFiberRoot: () => {},
+						onCommitFiberRoot: (_rendererId, root) => {
+							collectRenderedComponentsFromCommit(root)
+						},
 						onCommitFiberUnmount: () => {},
 					}
 				}
@@ -45,19 +116,32 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 						throw new Error('🚨 React DevTools is not available')
 					}
 
-					internals.enableProfilerTimer = true
-					internals.enableProfilerCommitHooks = true
-					internals.injectProfilingHooks({
-						markComponentRenderStarted: (fiber) => {
-							componentNames.push(fiber.type.name || 'Anonymous')
-						},
-					})
-
-					await cb()
+					const supportsInjectedProfilingHooks =
+						typeof internals.injectProfilingHooks === 'function'
+
+					if (supportsInjectedProfilingHooks) {
+						internals.enableProfilerTimer = true
+						internals.enableProfilerCommitHooks = true
+						internals.injectProfilingHooks({
+							markComponentRenderStarted: (fiber) => {
+								componentNames.push(fiber.type.name || 'Anonymous')
+							},
+						})
+					} else {
+						activeFallbackComponentNames = componentNames
+					}
 
-					internals.enableProfilerTimer = false
-					internals.enableProfilerCommitHooks = false
-					internals.injectProfilingHooks(null)
+					try {
+						await cb()
+					} finally {
+						if (supportsInjectedProfilingHooks) {
+							internals.enableProfilerTimer = false
+							internals.enableProfilerCommitHooks = false
+							internals.injectProfilingHooks(null)
+						} else {
+							activeFallbackComponentNames = null
+						}
+					}
 
 					return componentNames
 				}
diff --git a/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts b/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
index 8f5ee8f83..67806a246 100644
--- a/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
+++ b/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
@@ -12,9 +12,72 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 		const scriptToInject = `
 			<script>
 				let internals
+				const PerformedWorkFlag = 0b000000000000000000000000001
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+
+				let activeFallbackComponentNames = null
+
+				function isUserCodeFiberTag(tag) {
+					return UserCodeFiberTags.has(tag)
+				}
+
+				function getDisplayNameFromType(type) {
+					if (!type) return null
+
+					if (typeof type === 'function') {
+						return type.displayName || type.name || null
+					}
+
+					if (typeof type === 'object') {
+						if (typeof type.displayName === 'string' && type.displayName) {
+							return type.displayName
+						}
+
+						if (typeof type.render === 'function') {
+							return type.render.displayName || type.render.name || null
+						}
+
+						if (type.type) {
+							return getDisplayNameFromType(type.type)
+						}
+					}
+
+					return null
+				}
+
+				function getFiberDisplayName(fiber) {
+					return getDisplayNameFromType(fiber.type) || getDisplayNameFromType(fiber.elementType)
+				}
+
+				function didFiberRender(previousFiber, nextFiber) {
+					if (!previousFiber) return true
+					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
+				}
+
+				function collectRenderedComponentsFromCommit(root) {
+					if (!activeFallbackComponentNames || !root?.current) return
+
+					const stack = [root.current]
+					while (stack.length > 0) {
+						const fiber = stack.pop()
+						if (!fiber) continue
+
+						if (fiber.sibling) stack.push(fiber.sibling)
+						if (fiber.child) stack.push(fiber.child)
+
+						if (!isUserCodeFiberTag(fiber.tag)) continue
+						if (!didFiberRender(fiber.alternate, fiber)) continue
+
+						const componentName = getFiberDisplayName(fiber)
+						if (componentName) {
+							activeFallbackComponentNames.push(componentName)
+						}
+					}
+				}
 
 				function enhanceExistingHook(existingHook) {
 					const originalInject = existingHook.inject
+					const originalCommitFiberRoot = existingHook.onCommitFiberRoot
 
 					existingHook.inject = (injectedInternals) => {
 						internals = injectedInternals
@@ -23,6 +86,12 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 						return originalInject?.call(existingHook, injectedInternals) ?? 1
 					}
 
+					existingHook.onCommitFiberRoot = (...args) => {
+						const [, root] = args
+						collectRenderedComponentsFromCommit(root)
+						return originalCommitFiberRoot?.apply(existingHook, args)
+					}
+
 					return existingHook
 				}
 
@@ -34,7 +103,9 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 							internals = injectedInternals
 							return 1 // Returning a number as React expects a renderer ID
 						},
-						onCommitFiberRoot: () => {},
+						onCommitFiberRoot: (_rendererId, root) => {
+							collectRenderedComponentsFromCommit(root)
+						},
 						onCommitFiberUnmount: () => {},
 					}
 				}
@@ -45,19 +116,32 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 						throw new Error('🚨 React DevTools is not available')
 					}
 
-					internals.enableProfilerTimer = true
-					internals.enableProfilerCommitHooks = true
-					internals.injectProfilingHooks({
-						markComponentRenderStarted: (fiber) => {
-							componentNames.push(fiber.type.name || 'Anonymous')
-						},
-					})
-
-					await cb()
+					const supportsInjectedProfilingHooks =
+						typeof internals.injectProfilingHooks === 'function'
+
+					if (supportsInjectedProfilingHooks) {
+						internals.enableProfilerTimer = true
+						internals.enableProfilerCommitHooks = true
+						internals.injectProfilingHooks({
+							markComponentRenderStarted: (fiber) => {
+								componentNames.push(fiber.type.name || 'Anonymous')
+							},
+						})
+					} else {
+						activeFallbackComponentNames = componentNames
+					}
 
-					internals.enableProfilerTimer = false
-					internals.enableProfilerCommitHooks = false
-					internals.injectProfilingHooks(null)
+					try {
+						await cb()
+					} finally {
+						if (supportsInjectedProfilingHooks) {
+							internals.enableProfilerTimer = false
+							internals.enableProfilerCommitHooks = false
+							internals.injectProfilingHooks(null)
+						} else {
+							activeFallbackComponentNames = null
+						}
+					}
 
 					return componentNames
 				}
diff --git a/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts b/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
index a1025d863..50ad31452 100644
--- a/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
+++ b/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
@@ -12,9 +12,72 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 		const scriptToInject = `
 			<script>
 				let internals
+				const PerformedWorkFlag = 0b000000000000000000000000001
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+
+				let activeFallbackComponentNames = null
+
+				function isUserCodeFiberTag(tag) {
+					return UserCodeFiberTags.has(tag)
+				}
+
+				function getDisplayNameFromType(type) {
+					if (!type) return null
+
+					if (typeof type === 'function') {
+						return type.displayName || type.name || null
+					}
+
+					if (typeof type === 'object') {
+						if (typeof type.displayName === 'string' && type.displayName) {
+							return type.displayName
+						}
+
+						if (typeof type.render === 'function') {
+							return type.render.displayName || type.render.name || null
+						}
+
+						if (type.type) {
+							return getDisplayNameFromType(type.type)
+						}
+					}
+
+					return null
+				}
+
+				function getFiberDisplayName(fiber) {
+					return getDisplayNameFromType(fiber.type) || getDisplayNameFromType(fiber.elementType)
+				}
+
+				function didFiberRender(previousFiber, nextFiber) {
+					if (!previousFiber) return true
+					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
+				}
+
+				function collectRenderedComponentsFromCommit(root) {
+					if (!activeFallbackComponentNames || !root?.current) return
+
+					const stack = [root.current]
+					while (stack.length > 0) {
+						const fiber = stack.pop()
+						if (!fiber) continue
+
+						if (fiber.sibling) stack.push(fiber.sibling)
+						if (fiber.child) stack.push(fiber.child)
+
+						if (!isUserCodeFiberTag(fiber.tag)) continue
+						if (!didFiberRender(fiber.alternate, fiber)) continue
+
+						const componentName = getFiberDisplayName(fiber)
+						if (componentName) {
+							activeFallbackComponentNames.push(componentName)
+						}
+					}
+				}
 
 				function enhanceExistingHook(existingHook) {
 					const originalInject = existingHook.inject
+					const originalCommitFiberRoot = existingHook.onCommitFiberRoot
 
 					existingHook.inject = (injectedInternals) => {
 						internals = injectedInternals
@@ -23,6 +86,12 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						return originalInject?.call(existingHook, injectedInternals) ?? 1
 					}
 
+					existingHook.onCommitFiberRoot = (...args) => {
+						const [, root] = args
+						collectRenderedComponentsFromCommit(root)
+						return originalCommitFiberRoot?.apply(existingHook, args)
+					}
+
 					return existingHook
 				}
 
@@ -34,7 +103,9 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 							internals = injectedInternals
 							return 1 // Returning a number as React expects a renderer ID
 						},
-						onCommitFiberRoot: () => {},
+						onCommitFiberRoot: (_rendererId, root) => {
+							collectRenderedComponentsFromCommit(root)
+						},
 						onCommitFiberUnmount: () => {},
 					}
 				}
@@ -45,19 +116,32 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						throw new Error('🚨 React DevTools is not available')
 					}
 
-					internals.enableProfilerTimer = true
-					internals.enableProfilerCommitHooks = true
-					internals.injectProfilingHooks({
-						markComponentRenderStarted: (fiber) => {
-							componentNames.push(fiber.type.name || 'Anonymous')
-						},
-					})
-
-					await cb()
+					const supportsInjectedProfilingHooks =
+						typeof internals.injectProfilingHooks === 'function'
+
+					if (supportsInjectedProfilingHooks) {
+						internals.enableProfilerTimer = true
+						internals.enableProfilerCommitHooks = true
+						internals.injectProfilingHooks({
+							markComponentRenderStarted: (fiber) => {
+								componentNames.push(fiber.type.name || 'Anonymous')
+							},
+						})
+					} else {
+						activeFallbackComponentNames = componentNames
+					}
 
-					internals.enableProfilerTimer = false
-					internals.enableProfilerCommitHooks = false
-					internals.injectProfilingHooks(null)
+					try {
+						await cb()
+					} finally {
+						if (supportsInjectedProfilingHooks) {
+							internals.enableProfilerTimer = false
+							internals.enableProfilerCommitHooks = false
+							internals.injectProfilingHooks(null)
+						} else {
+							activeFallbackComponentNames = null
+						}
+					}
 
 					return componentNames
 				}
diff --git a/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts b/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
index a1025d863..50ad31452 100644
--- a/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
+++ b/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
@@ -12,9 +12,72 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 		const scriptToInject = `
 			<script>
 				let internals
+				const PerformedWorkFlag = 0b000000000000000000000000001
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+
+				let activeFallbackComponentNames = null
+
+				function isUserCodeFiberTag(tag) {
+					return UserCodeFiberTags.has(tag)
+				}
+
+				function getDisplayNameFromType(type) {
+					if (!type) return null
+
+					if (typeof type === 'function') {
+						return type.displayName || type.name || null
+					}
+
+					if (typeof type === 'object') {
+						if (typeof type.displayName === 'string' && type.displayName) {
+							return type.displayName
+						}
+
+						if (typeof type.render === 'function') {
+							return type.render.displayName || type.render.name || null
+						}
+
+						if (type.type) {
+							return getDisplayNameFromType(type.type)
+						}
+					}
+
+					return null
+				}
+
+				function getFiberDisplayName(fiber) {
+					return getDisplayNameFromType(fiber.type) || getDisplayNameFromType(fiber.elementType)
+				}
+
+				function didFiberRender(previousFiber, nextFiber) {
+					if (!previousFiber) return true
+					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
+				}
+
+				function collectRenderedComponentsFromCommit(root) {
+					if (!activeFallbackComponentNames || !root?.current) return
+
+					const stack = [root.current]
+					while (stack.length > 0) {
+						const fiber = stack.pop()
+						if (!fiber) continue
+
+						if (fiber.sibling) stack.push(fiber.sibling)
+						if (fiber.child) stack.push(fiber.child)
+
+						if (!isUserCodeFiberTag(fiber.tag)) continue
+						if (!didFiberRender(fiber.alternate, fiber)) continue
+
+						const componentName = getFiberDisplayName(fiber)
+						if (componentName) {
+							activeFallbackComponentNames.push(componentName)
+						}
+					}
+				}
 
 				function enhanceExistingHook(existingHook) {
 					const originalInject = existingHook.inject
+					const originalCommitFiberRoot = existingHook.onCommitFiberRoot
 
 					existingHook.inject = (injectedInternals) => {
 						internals = injectedInternals
@@ -23,6 +86,12 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						return originalInject?.call(existingHook, injectedInternals) ?? 1
 					}
 
+					existingHook.onCommitFiberRoot = (...args) => {
+						const [, root] = args
+						collectRenderedComponentsFromCommit(root)
+						return originalCommitFiberRoot?.apply(existingHook, args)
+					}
+
 					return existingHook
 				}
 
@@ -34,7 +103,9 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 							internals = injectedInternals
 							return 1 // Returning a number as React expects a renderer ID
 						},
-						onCommitFiberRoot: () => {},
+						onCommitFiberRoot: (_rendererId, root) => {
+							collectRenderedComponentsFromCommit(root)
+						},
 						onCommitFiberUnmount: () => {},
 					}
 				}
@@ -45,19 +116,32 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						throw new Error('🚨 React DevTools is not available')
 					}
 
-					internals.enableProfilerTimer = true
-					internals.enableProfilerCommitHooks = true
-					internals.injectProfilingHooks({
-						markComponentRenderStarted: (fiber) => {
-							componentNames.push(fiber.type.name || 'Anonymous')
-						},
-					})
-
-					await cb()
+					const supportsInjectedProfilingHooks =
+						typeof internals.injectProfilingHooks === 'function'
+
+					if (supportsInjectedProfilingHooks) {
+						internals.enableProfilerTimer = true
+						internals.enableProfilerCommitHooks = true
+						internals.injectProfilingHooks({
+							markComponentRenderStarted: (fiber) => {
+								componentNames.push(fiber.type.name || 'Anonymous')
+							},
+						})
+					} else {
+						activeFallbackComponentNames = componentNames
+					}
 
-					internals.enableProfilerTimer = false
-					internals.enableProfilerCommitHooks = false
-					internals.injectProfilingHooks(null)
+					try {
+						await cb()
+					} finally {
+						if (supportsInjectedProfilingHooks) {
+							internals.enableProfilerTimer = false
+							internals.enableProfilerCommitHooks = false
+							internals.injectProfilingHooks(null)
+						} else {
+							activeFallbackComponentNames = null
+						}
+					}
 
 					return componentNames
 				}
diff --git a/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts b/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
index a1025d863..50ad31452 100644
--- a/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
+++ b/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
@@ -12,9 +12,72 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 		const scriptToInject = `
 			<script>
 				let internals
+				const PerformedWorkFlag = 0b000000000000000000000000001
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+
+				let activeFallbackComponentNames = null
+
+				function isUserCodeFiberTag(tag) {
+					return UserCodeFiberTags.has(tag)
+				}
+
+				function getDisplayNameFromType(type) {
+					if (!type) return null
+
+					if (typeof type === 'function') {
+						return type.displayName || type.name || null
+					}
+
+					if (typeof type === 'object') {
+						if (typeof type.displayName === 'string' && type.displayName) {
+							return type.displayName
+						}
+
+						if (typeof type.render === 'function') {
+							return type.render.displayName || type.render.name || null
+						}
+
+						if (type.type) {
+							return getDisplayNameFromType(type.type)
+						}
+					}
+
+					return null
+				}
+
+				function getFiberDisplayName(fiber) {
+					return getDisplayNameFromType(fiber.type) || getDisplayNameFromType(fiber.elementType)
+				}
+
+				function didFiberRender(previousFiber, nextFiber) {
+					if (!previousFiber) return true
+					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
+				}
+
+				function collectRenderedComponentsFromCommit(root) {
+					if (!activeFallbackComponentNames || !root?.current) return
+
+					const stack = [root.current]
+					while (stack.length > 0) {
+						const fiber = stack.pop()
+						if (!fiber) continue
+
+						if (fiber.sibling) stack.push(fiber.sibling)
+						if (fiber.child) stack.push(fiber.child)
+
+						if (!isUserCodeFiberTag(fiber.tag)) continue
+						if (!didFiberRender(fiber.alternate, fiber)) continue
+
+						const componentName = getFiberDisplayName(fiber)
+						if (componentName) {
+							activeFallbackComponentNames.push(componentName)
+						}
+					}
+				}
 
 				function enhanceExistingHook(existingHook) {
 					const originalInject = existingHook.inject
+					const originalCommitFiberRoot = existingHook.onCommitFiberRoot
 
 					existingHook.inject = (injectedInternals) => {
 						internals = injectedInternals
@@ -23,6 +86,12 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						return originalInject?.call(existingHook, injectedInternals) ?? 1
 					}
 
+					existingHook.onCommitFiberRoot = (...args) => {
+						const [, root] = args
+						collectRenderedComponentsFromCommit(root)
+						return originalCommitFiberRoot?.apply(existingHook, args)
+					}
+
 					return existingHook
 				}
 
@@ -34,7 +103,9 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 							internals = injectedInternals
 							return 1 // Returning a number as React expects a renderer ID
 						},
-						onCommitFiberRoot: () => {},
+						onCommitFiberRoot: (_rendererId, root) => {
+							collectRenderedComponentsFromCommit(root)
+						},
 						onCommitFiberUnmount: () => {},
 					}
 				}
@@ -45,19 +116,32 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						throw new Error('🚨 React DevTools is not available')
 					}
 
-					internals.enableProfilerTimer = true
-					internals.enableProfilerCommitHooks = true
-					internals.injectProfilingHooks({
-						markComponentRenderStarted: (fiber) => {
-							componentNames.push(fiber.type.name || 'Anonymous')
-						},
-					})
-
-					await cb()
+					const supportsInjectedProfilingHooks =
+						typeof internals.injectProfilingHooks === 'function'
+
+					if (supportsInjectedProfilingHooks) {
+						internals.enableProfilerTimer = true
+						internals.enableProfilerCommitHooks = true
+						internals.injectProfilingHooks({
+							markComponentRenderStarted: (fiber) => {
+								componentNames.push(fiber.type.name || 'Anonymous')
+							},
+						})
+					} else {
+						activeFallbackComponentNames = componentNames
+					}
 
-					internals.enableProfilerTimer = false
-					internals.enableProfilerCommitHooks = false
-					internals.injectProfilingHooks(null)
+					try {
+						await cb()
+					} finally {
+						if (supportsInjectedProfilingHooks) {
+							internals.enableProfilerTimer = false
+							internals.enableProfilerCommitHooks = false
+							internals.injectProfilingHooks(null)
+						} else {
+							activeFallbackComponentNames = null
+						}
+					}
 
 					return componentNames
 				}
diff --git a/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts b/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
index a1025d863..50ad31452 100644
--- a/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
+++ b/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
@@ -12,9 +12,72 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 		const scriptToInject = `
 			<script>
 				let internals
+				const PerformedWorkFlag = 0b000000000000000000000000001
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+
+				let activeFallbackComponentNames = null
+
+				function isUserCodeFiberTag(tag) {
+					return UserCodeFiberTags.has(tag)
+				}
+
+				function getDisplayNameFromType(type) {
+					if (!type) return null
+
+					if (typeof type === 'function') {
+						return type.displayName || type.name || null
+					}
+
+					if (typeof type === 'object') {
+						if (typeof type.displayName === 'string' && type.displayName) {
+							return type.displayName
+						}
+
+						if (typeof type.render === 'function') {
+							return type.render.displayName || type.render.name || null
+						}
+
+						if (type.type) {
+							return getDisplayNameFromType(type.type)
+						}
+					}
+
+					return null
+				}
+
+				function getFiberDisplayName(fiber) {
+					return getDisplayNameFromType(fiber.type) || getDisplayNameFromType(fiber.elementType)
+				}
+
+				function didFiberRender(previousFiber, nextFiber) {
+					if (!previousFiber) return true
+					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
+				}
+
+				function collectRenderedComponentsFromCommit(root) {
+					if (!activeFallbackComponentNames || !root?.current) return
+
+					const stack = [root.current]
+					while (stack.length > 0) {
+						const fiber = stack.pop()
+						if (!fiber) continue
+
+						if (fiber.sibling) stack.push(fiber.sibling)
+						if (fiber.child) stack.push(fiber.child)
+
+						if (!isUserCodeFiberTag(fiber.tag)) continue
+						if (!didFiberRender(fiber.alternate, fiber)) continue
+
+						const componentName = getFiberDisplayName(fiber)
+						if (componentName) {
+							activeFallbackComponentNames.push(componentName)
+						}
+					}
+				}
 
 				function enhanceExistingHook(existingHook) {
 					const originalInject = existingHook.inject
+					const originalCommitFiberRoot = existingHook.onCommitFiberRoot
 
 					existingHook.inject = (injectedInternals) => {
 						internals = injectedInternals
@@ -23,6 +86,12 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						return originalInject?.call(existingHook, injectedInternals) ?? 1
 					}
 
+					existingHook.onCommitFiberRoot = (...args) => {
+						const [, root] = args
+						collectRenderedComponentsFromCommit(root)
+						return originalCommitFiberRoot?.apply(existingHook, args)
+					}
+
 					return existingHook
 				}
 
@@ -34,7 +103,9 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 							internals = injectedInternals
 							return 1 // Returning a number as React expects a renderer ID
 						},
-						onCommitFiberRoot: () => {},
+						onCommitFiberRoot: (_rendererId, root) => {
+							collectRenderedComponentsFromCommit(root)
+						},
 						onCommitFiberUnmount: () => {},
 					}
 				}
@@ -45,19 +116,32 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						throw new Error('🚨 React DevTools is not available')
 					}
 
-					internals.enableProfilerTimer = true
-					internals.enableProfilerCommitHooks = true
-					internals.injectProfilingHooks({
-						markComponentRenderStarted: (fiber) => {
-							componentNames.push(fiber.type.name || 'Anonymous')
-						},
-					})
-
-					await cb()
+					const supportsInjectedProfilingHooks =
+						typeof internals.injectProfilingHooks === 'function'
+
+					if (supportsInjectedProfilingHooks) {
+						internals.enableProfilerTimer = true
+						internals.enableProfilerCommitHooks = true
+						internals.injectProfilingHooks({
+							markComponentRenderStarted: (fiber) => {
+								componentNames.push(fiber.type.name || 'Anonymous')
+							},
+						})
+					} else {
+						activeFallbackComponentNames = componentNames
+					}
 
-					internals.enableProfilerTimer = false
-					internals.enableProfilerCommitHooks = false
-					internals.injectProfilingHooks(null)
+					try {
+						await cb()
+					} finally {
+						if (supportsInjectedProfilingHooks) {
+							internals.enableProfilerTimer = false
+							internals.enableProfilerCommitHooks = false
+							internals.injectProfilingHooks(null)
+						} else {
+							activeFallbackComponentNames = null
+						}
+					}
 
 					return componentNames
 				}
diff --git a/shared/get-component-calls.tsx b/shared/get-component-calls.tsx
index c2b17afbf..8b9d73b2d 100644
--- a/shared/get-component-calls.tsx
+++ b/shared/get-component-calls.tsx
@@ -6,8 +6,84 @@ declare global {
 
 let internals: any
 
+const PerformedWorkFlag = 0b000000000000000000000000001
+const FunctionComponentTag = 0
+const ClassComponentTag = 1
+const ContextConsumerTag = 9
+const ForwardRefTag = 11
+const MemoComponentTag = 14
+const SimpleMemoComponentTag = 15
+
+let activeFallbackComponentNames: Array<string> | null = null
+
+function isUserCodeFiberTag(tag: number) {
+	return (
+		tag === FunctionComponentTag ||
+		tag === ClassComponentTag ||
+		tag === ContextConsumerTag ||
+		tag === ForwardRefTag ||
+		tag === MemoComponentTag ||
+		tag === SimpleMemoComponentTag
+	)
+}
+
+function getDisplayNameFromType(type: any): string | null {
+	if (!type) return null
+
+	if (typeof type === 'function') {
+		return type.displayName || type.name || null
+	}
+
+	if (typeof type === 'object') {
+		if (typeof type.displayName === 'string' && type.displayName) {
+			return type.displayName
+		}
+
+		if (typeof type.render === 'function') {
+			return type.render.displayName || type.render.name || null
+		}
+
+		if (type.type) {
+			return getDisplayNameFromType(type.type)
+		}
+	}
+
+	return null
+}
+
+function getFiberDisplayName(fiber: any) {
+	return getDisplayNameFromType(fiber.type) || getDisplayNameFromType(fiber.elementType)
+}
+
+function didFiberRender(previousFiber: any, nextFiber: any) {
+	if (!previousFiber) return true
+	return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
+}
+
+function collectRenderedComponentsFromCommit(root: any) {
+	if (!activeFallbackComponentNames || !root?.current) return
+
+	const stack = [root.current]
+	while (stack.length > 0) {
+		const fiber = stack.pop()
+		if (!fiber) continue
+
+		if (fiber.sibling) stack.push(fiber.sibling)
+		if (fiber.child) stack.push(fiber.child)
+
+		if (!isUserCodeFiberTag(fiber.tag)) continue
+		if (!didFiberRender(fiber.alternate, fiber)) continue
+
+		const componentName = getFiberDisplayName(fiber)
+		if (componentName) {
+			activeFallbackComponentNames.push(componentName)
+		}
+	}
+}
+
 function enhanceExistingHook(existingHook: any) {
 	const originalInject = existingHook.inject
+	const originalCommitFiberRoot = existingHook.onCommitFiberRoot
 
 	existingHook.inject = (injectedInternals: any) => {
 		internals = injectedInternals
@@ -16,6 +92,12 @@ function enhanceExistingHook(existingHook: any) {
 		return originalInject?.call(existingHook, injectedInternals) ?? 1
 	}
 
+	existingHook.onCommitFiberRoot = (...args: Array<any>) => {
+		const [, root] = args
+		collectRenderedComponentsFromCommit(root)
+		return originalCommitFiberRoot?.apply(existingHook, args)
+	}
+
 	return existingHook
 }
 
@@ -26,7 +108,9 @@ function createMinimalHook() {
 			internals = injectedInternals
 			return 1 // Returning a number as React expects a renderer ID
 		},
-		onCommitFiberRoot: () => {},
+		onCommitFiberRoot: (_rendererId: number, root: any) => {
+			collectRenderedComponentsFromCommit(root)
+		},
 		onCommitFiberUnmount: () => {},
 	}
 }
@@ -37,19 +121,32 @@ export async function getComponentCalls(cb: () => void | Promise<void>) {
 		throw new Error('🚨 React DevTools is not available')
 	}
 
-	internals.enableProfilerTimer = true
-	internals.enableProfilerCommitHooks = true
-	internals.injectProfilingHooks({
-		markComponentRenderStarted: (fiber: any) => {
-			componentNames.push(fiber.type.name || 'Anonymous')
-		},
-	})
+	const supportsInjectedProfilingHooks =
+		typeof internals.injectProfilingHooks === 'function'
 
-	await cb()
+	if (supportsInjectedProfilingHooks) {
+		internals.enableProfilerTimer = true
+		internals.enableProfilerCommitHooks = true
+		internals.injectProfilingHooks({
+			markComponentRenderStarted: (fiber: any) => {
+				componentNames.push(fiber.type.name || 'Anonymous')
+			},
+		})
+	} else {
+		activeFallbackComponentNames = componentNames
+	}
 
-	internals.enableProfilerTimer = false
-	internals.enableProfilerCommitHooks = false
-	internals.injectProfilingHooks(null)
+	try {
+		await cb()
+	} finally {
+		if (supportsInjectedProfilingHooks) {
+			internals.enableProfilerTimer = false
+			internals.enableProfilerCommitHooks = false
+			internals.injectProfilingHooks(null)
+		} else {
+			activeFallbackComponentNames = null
+		}
+	}
 
 	return componentNames
 }

From 9bcb800a421d3d671dd132a8b7ff9416a47e16b0 Mon Sep 17 00:00:00 2001
From: Cursor Agent <cursoragent@cursor.com>
Date: Tue, 24 Feb 2026 22:05:45 +0000
Subject: [PATCH 2/5] Refine fallback render detection for memoized fibers

Co-authored-by: Kent C. Dodds <me+github@kentcdodds.com>
---
 exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts | 4 ++--
 .../06.rerenders/01.solution.memo/tests/memoized.test.ts      | 4 ++--
 .../06.rerenders/02.problem.comparator/tests/memoized.test.ts | 4 ++--
 .../02.solution.comparator/tests/memoized.test.ts             | 4 ++--
 .../06.rerenders/03.problem.primitives/tests/memoized.test.ts | 4 ++--
 .../03.solution.primitives/tests/memoized.test.ts             | 4 ++--
 shared/get-component-calls.tsx                                | 4 +---
 7 files changed, 13 insertions(+), 15 deletions(-)

diff --git a/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts b/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
index 67806a246..f0f9f9b1d 100644
--- a/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
+++ b/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
@@ -13,7 +13,7 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 			<script>
 				let internals
 				const PerformedWorkFlag = 0b000000000000000000000000001
-				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 15])
 
 				let activeFallbackComponentNames = null
 
@@ -50,7 +50,7 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return true
+					if (!previousFiber) return false
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts b/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
index 67806a246..f0f9f9b1d 100644
--- a/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
+++ b/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
@@ -13,7 +13,7 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 			<script>
 				let internals
 				const PerformedWorkFlag = 0b000000000000000000000000001
-				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 15])
 
 				let activeFallbackComponentNames = null
 
@@ -50,7 +50,7 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return true
+					if (!previousFiber) return false
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts b/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
index 50ad31452..4e02ac84b 100644
--- a/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
+++ b/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
@@ -13,7 +13,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 			<script>
 				let internals
 				const PerformedWorkFlag = 0b000000000000000000000000001
-				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 15])
 
 				let activeFallbackComponentNames = null
 
@@ -50,7 +50,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return true
+					if (!previousFiber) return false
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts b/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
index 50ad31452..4e02ac84b 100644
--- a/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
+++ b/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
@@ -13,7 +13,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 			<script>
 				let internals
 				const PerformedWorkFlag = 0b000000000000000000000000001
-				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 15])
 
 				let activeFallbackComponentNames = null
 
@@ -50,7 +50,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return true
+					if (!previousFiber) return false
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts b/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
index 50ad31452..4e02ac84b 100644
--- a/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
+++ b/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
@@ -13,7 +13,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 			<script>
 				let internals
 				const PerformedWorkFlag = 0b000000000000000000000000001
-				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 15])
 
 				let activeFallbackComponentNames = null
 
@@ -50,7 +50,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return true
+					if (!previousFiber) return false
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts b/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
index 50ad31452..4e02ac84b 100644
--- a/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
+++ b/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
@@ -13,7 +13,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 			<script>
 				let internals
 				const PerformedWorkFlag = 0b000000000000000000000000001
-				const UserCodeFiberTags = new Set([0, 1, 9, 11, 14, 15])
+				const UserCodeFiberTags = new Set([0, 1, 9, 11, 15])
 
 				let activeFallbackComponentNames = null
 
@@ -50,7 +50,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return true
+					if (!previousFiber) return false
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/shared/get-component-calls.tsx b/shared/get-component-calls.tsx
index 8b9d73b2d..07ccb6bcc 100644
--- a/shared/get-component-calls.tsx
+++ b/shared/get-component-calls.tsx
@@ -11,7 +11,6 @@ const FunctionComponentTag = 0
 const ClassComponentTag = 1
 const ContextConsumerTag = 9
 const ForwardRefTag = 11
-const MemoComponentTag = 14
 const SimpleMemoComponentTag = 15
 
 let activeFallbackComponentNames: Array<string> | null = null
@@ -22,7 +21,6 @@ function isUserCodeFiberTag(tag: number) {
 		tag === ClassComponentTag ||
 		tag === ContextConsumerTag ||
 		tag === ForwardRefTag ||
-		tag === MemoComponentTag ||
 		tag === SimpleMemoComponentTag
 	)
 }
@@ -56,7 +54,7 @@ function getFiberDisplayName(fiber: any) {
 }
 
 function didFiberRender(previousFiber: any, nextFiber: any) {
-	if (!previousFiber) return true
+	if (!previousFiber) return false
 	return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 }
 

From fd1af291b3f426731611700766d5641db83900e7 Mon Sep 17 00:00:00 2001
From: Cursor Agent <cursoragent@cursor.com>
Date: Tue, 24 Feb 2026 23:03:06 +0000
Subject: [PATCH 3/5] Count initial mounts in fallback render tracking

---
 exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts   | 2 +-
 exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts  | 2 +-
 .../06.rerenders/02.problem.comparator/tests/memoized.test.ts   | 2 +-
 .../06.rerenders/02.solution.comparator/tests/memoized.test.ts  | 2 +-
 .../06.rerenders/03.problem.primitives/tests/memoized.test.ts   | 2 +-
 .../06.rerenders/03.solution.primitives/tests/memoized.test.ts  | 2 +-
 shared/get-component-calls.tsx                                  | 2 +-
 7 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts b/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
index f0f9f9b1d..d50cba9db 100644
--- a/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
+++ b/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
@@ -50,7 +50,7 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return false
+					if (!previousFiber) return true
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts b/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
index f0f9f9b1d..d50cba9db 100644
--- a/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
+++ b/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
@@ -50,7 +50,7 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return false
+					if (!previousFiber) return true
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts b/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
index 4e02ac84b..7e7a14b1d 100644
--- a/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
+++ b/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
@@ -50,7 +50,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return false
+					if (!previousFiber) return true
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts b/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
index 4e02ac84b..7e7a14b1d 100644
--- a/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
+++ b/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
@@ -50,7 +50,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return false
+					if (!previousFiber) return true
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts b/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
index 4e02ac84b..7e7a14b1d 100644
--- a/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
+++ b/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
@@ -50,7 +50,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return false
+					if (!previousFiber) return true
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts b/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
index 4e02ac84b..7e7a14b1d 100644
--- a/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
+++ b/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
@@ -50,7 +50,7 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 				}
 
 				function didFiberRender(previousFiber, nextFiber) {
-					if (!previousFiber) return false
+					if (!previousFiber) return true
 					return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 				}
 
diff --git a/shared/get-component-calls.tsx b/shared/get-component-calls.tsx
index 07ccb6bcc..b9b60067c 100644
--- a/shared/get-component-calls.tsx
+++ b/shared/get-component-calls.tsx
@@ -54,7 +54,7 @@ function getFiberDisplayName(fiber: any) {
 }
 
 function didFiberRender(previousFiber: any, nextFiber: any) {
-	if (!previousFiber) return false
+	if (!previousFiber) return true
 	return (nextFiber.flags & PerformedWorkFlag) === PerformedWorkFlag
 }
 

From 3d929eb184532a45110edafbdad0e8b943e479c4 Mon Sep 17 00:00:00 2001
From: Cursor Agent <cursoragent@cursor.com>
Date: Tue, 24 Feb 2026 23:16:32 +0000
Subject: [PATCH 4/5] Fix fallback anonymous component naming

---
 shared/get-component-calls.tsx | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/shared/get-component-calls.tsx b/shared/get-component-calls.tsx
index b9b60067c..4079a2113 100644
--- a/shared/get-component-calls.tsx
+++ b/shared/get-component-calls.tsx
@@ -72,10 +72,8 @@ function collectRenderedComponentsFromCommit(root: any) {
 		if (!isUserCodeFiberTag(fiber.tag)) continue
 		if (!didFiberRender(fiber.alternate, fiber)) continue
 
-		const componentName = getFiberDisplayName(fiber)
-		if (componentName) {
-			activeFallbackComponentNames.push(componentName)
-		}
+		const componentName = getFiberDisplayName(fiber) ?? 'Anonymous'
+		activeFallbackComponentNames.push(componentName)
 	}
 }
 

From bcb217127756b3658523af509f9ef755cf3e2f34 Mon Sep 17 00:00:00 2001
From: Cursor Agent <cursoragent@cursor.com>
Date: Tue, 24 Feb 2026 23:23:52 +0000
Subject: [PATCH 5/5] Handle anonymous components in fallback

---
 .../06.rerenders/01.problem.memo/tests/memoized.test.ts     | 6 ++----
 .../06.rerenders/01.solution.memo/tests/memoized.test.ts    | 6 ++----
 .../02.problem.comparator/tests/memoized.test.ts            | 6 ++----
 .../02.solution.comparator/tests/memoized.test.ts           | 6 ++----
 .../03.problem.primitives/tests/memoized.test.ts            | 6 ++----
 .../03.solution.primitives/tests/memoized.test.ts           | 6 ++----
 6 files changed, 12 insertions(+), 24 deletions(-)

diff --git a/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts b/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
index d50cba9db..6000aea52 100644
--- a/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
+++ b/exercises/06.rerenders/01.problem.memo/tests/memoized.test.ts
@@ -68,10 +68,8 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 						if (!isUserCodeFiberTag(fiber.tag)) continue
 						if (!didFiberRender(fiber.alternate, fiber)) continue
 
-						const componentName = getFiberDisplayName(fiber)
-						if (componentName) {
-							activeFallbackComponentNames.push(componentName)
-						}
+						const componentName = getFiberDisplayName(fiber) ?? 'Anonymous'
+						activeFallbackComponentNames.push(componentName)
 					}
 				}
 
diff --git a/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts b/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
index d50cba9db..6000aea52 100644
--- a/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
+++ b/exercises/06.rerenders/01.solution.memo/tests/memoized.test.ts
@@ -68,10 +68,8 @@ test('Only ListItem should not rerender when clicking force rerender', async ({
 						if (!isUserCodeFiberTag(fiber.tag)) continue
 						if (!didFiberRender(fiber.alternate, fiber)) continue
 
-						const componentName = getFiberDisplayName(fiber)
-						if (componentName) {
-							activeFallbackComponentNames.push(componentName)
-						}
+						const componentName = getFiberDisplayName(fiber) ?? 'Anonymous'
+						activeFallbackComponentNames.push(componentName)
 					}
 				}
 
diff --git a/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts b/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
index 7e7a14b1d..4e267d303 100644
--- a/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
+++ b/exercises/06.rerenders/02.problem.comparator/tests/memoized.test.ts
@@ -68,10 +68,8 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						if (!isUserCodeFiberTag(fiber.tag)) continue
 						if (!didFiberRender(fiber.alternate, fiber)) continue
 
-						const componentName = getFiberDisplayName(fiber)
-						if (componentName) {
-							activeFallbackComponentNames.push(componentName)
-						}
+						const componentName = getFiberDisplayName(fiber) ?? 'Anonymous'
+						activeFallbackComponentNames.push(componentName)
 					}
 				}
 
diff --git a/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts b/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
index 7e7a14b1d..4e267d303 100644
--- a/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
+++ b/exercises/06.rerenders/02.solution.comparator/tests/memoized.test.ts
@@ -68,10 +68,8 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						if (!isUserCodeFiberTag(fiber.tag)) continue
 						if (!didFiberRender(fiber.alternate, fiber)) continue
 
-						const componentName = getFiberDisplayName(fiber)
-						if (componentName) {
-							activeFallbackComponentNames.push(componentName)
-						}
+						const componentName = getFiberDisplayName(fiber) ?? 'Anonymous'
+						activeFallbackComponentNames.push(componentName)
 					}
 				}
 
diff --git a/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts b/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
index 7e7a14b1d..4e267d303 100644
--- a/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
+++ b/exercises/06.rerenders/03.problem.primitives/tests/memoized.test.ts
@@ -68,10 +68,8 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						if (!isUserCodeFiberTag(fiber.tag)) continue
 						if (!didFiberRender(fiber.alternate, fiber)) continue
 
-						const componentName = getFiberDisplayName(fiber)
-						if (componentName) {
-							activeFallbackComponentNames.push(componentName)
-						}
+						const componentName = getFiberDisplayName(fiber) ?? 'Anonymous'
+						activeFallbackComponentNames.push(componentName)
 					}
 				}
 
diff --git a/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts b/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
index 7e7a14b1d..4e267d303 100644
--- a/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
+++ b/exercises/06.rerenders/03.solution.primitives/tests/memoized.test.ts
@@ -68,10 +68,8 @@ test('Only two ListItems should not rerender when the highlighted item changes',
 						if (!isUserCodeFiberTag(fiber.tag)) continue
 						if (!didFiberRender(fiber.alternate, fiber)) continue
 
-						const componentName = getFiberDisplayName(fiber)
-						if (componentName) {
-							activeFallbackComponentNames.push(componentName)
-						}
+						const componentName = getFiberDisplayName(fiber) ?? 'Anonymous'
+						activeFallbackComponentNames.push(componentName)
 					}
 				}