From 7ca8a48318b99b64fdfedfe8a98074d3d0be2e0d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rub=C3=A9n=20Norte?= <rubennorte@meta.com>
Date: Mon, 26 Oct 2026 09:59:43 -0700
Subject: [PATCH] Warn only when Fantom root creation changes global dimensions

Summary:
Avoid repeated warnings when a new root matches the current global dimensions, even if older roots have different metrics. Continue warning when another root is alive and the new root changes the current metrics.

Changelog: [Internal] Compare Fantom root dimensions against current global metrics.

Differential Revision: D127606984
---
 .../react-native-fantom/__docs__/README.md    |  6 ++---
 .../src/__tests__/FantomDimensions-itest.js   |  7 ++++--
 private/react-native-fantom/src/index.js      | 25 +++++++++----------
 3 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/private/react-native-fantom/__docs__/README.md b/private/react-native-fantom/__docs__/README.md
index bb93983ac021..0a6d73d07e5d 100644
--- a/private/react-native-fantom/__docs__/README.md
+++ b/private/react-native-fantom/__docs__/README.md
@@ -95,9 +95,9 @@ describe('My feature', () => {
 Creating a root sets global `Dimensions` for both `window` and `screen` from its
 `viewportWidth`, `viewportHeight`, `devicePixelRatio`, and optional `fontScale`
 (default `1`). `useWindowDimensions()` and `PixelRatio` report the new root's
-values. If another root is still alive with different display metrics, Fantom
-applies the new values and emits a warning. Call `root.destroy()` when a root is
-no longer needed.
+values. If another root is still alive and the new metrics differ from the
+current global dimensions, Fantom applies the new values and emits a warning.
+Call `root.destroy()` when a root is no longer needed.
 
 For a full API reference, please see the [inline documentation](../src/index.js)
 defined for the methods in the `@react-native/fantom` [module](../src/index.js).
diff --git a/private/react-native-fantom/src/__tests__/FantomDimensions-itest.js b/private/react-native-fantom/src/__tests__/FantomDimensions-itest.js
index dc287128aff4..edb818f9aac6 100644
--- a/private/react-native-fantom/src/__tests__/FantomDimensions-itest.js
+++ b/private/react-native-fantom/src/__tests__/FantomDimensions-itest.js
@@ -116,12 +116,15 @@ for (const [name, config] of conflictingConfigs) {
   });
 }
 
-it('checks all active roots, including one that differs from the latest root', () => {
+it('compares new metrics only with the current global dimensions', () => {
   createRoot();
   createRoot({viewportWidth: 640});
   createRoot({viewportWidth: 640});
-  expect(warn).toHaveBeenCalledTimes(2);
+  expect(warn).toHaveBeenCalledTimes(1);
   expect(Dimensions.get('window').width).toBe(640);
+  createRoot();
+  expect(warn).toHaveBeenCalledTimes(2);
+  expect(Dimensions.get('window').width).toBe(390);
 });
 
 it('does not warn for a destroyed root', () => {
diff --git a/private/react-native-fantom/src/index.js b/private/react-native-fantom/src/index.js
index af451e6c2ab9..98937065dc6c 100644
--- a/private/react-native-fantom/src/index.js
+++ b/private/react-native-fantom/src/index.js
@@ -84,21 +84,20 @@ function pushDimensions(
   surfaceId: RootTag,
   displayMetrics: DisplayMetrics,
 ): void {
+  const previousDisplayMetrics = Dimensions.get('window');
   activeRootDisplayMetrics.set(surfaceId, displayMetrics);
   Dimensions.set({window: displayMetrics, screen: displayMetrics});
-  for (const metrics of activeRootDisplayMetrics.values()) {
-    if (
-      metrics.width !== displayMetrics.width ||
-      metrics.height !== displayMetrics.height ||
-      metrics.scale !== displayMetrics.scale ||
-      metrics.fontScale !== displayMetrics.fontScale
-    ) {
-      console.warn(
-        'Fantom.createRoot() updated global Dimensions while another active root has different display metrics. Dimensions and PixelRatio now reflect the new root.',
-        {existingRoot: metrics, newRoot: displayMetrics},
-      );
-      break;
-    }
+  if (
+    activeRootDisplayMetrics.size > 1 &&
+    (previousDisplayMetrics.width !== displayMetrics.width ||
+      previousDisplayMetrics.height !== displayMetrics.height ||
+      previousDisplayMetrics.scale !== displayMetrics.scale ||
+      previousDisplayMetrics.fontScale !== displayMetrics.fontScale)
+  ) {
+    console.warn(
+      'Fantom.createRoot() updated global Dimensions while another active root has different display metrics. Dimensions and PixelRatio now reflect the new root.',
+      {existingRoot: previousDisplayMetrics, newRoot: displayMetrics},
+    );
   }
 }