Fix text not scaling down when system fontScale < 1.0

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/uimanager/DisplayMetricsHolder.kt

@@ -73,6 +73,10 @@ public object DisplayMetricsHolder {
     // See:
     // http://developer.android.com/reference/android/view/Display.html#getRealMetrics(android.util.DisplayMetrics)
     @Suppress("DEPRECATION") wm.defaultDisplay.getRealMetrics(screenDisplayMetrics)
+    // Preserve fontScale from the configuration because getRealMetrics() returns
+    // physical display metrics without the system font scale setting.
+    // This is needed for proper text scaling when fontScale < 1.0
+    screenDisplayMetrics.scaledDensity = displayMetrics.scaledDensity
     DisplayMetricsHolder.screenDisplayMetrics = screenDisplayMetrics
   }
 

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/uimanager/DisplayMetricsHolderTest.kt

@@ -166,4 +166,18 @@ class DisplayMetricsHolderTest {
     assertThat(decodedWidth).isEqualTo(width)
     assertThat(decodedHeight).isEqualTo(height)
   }
+
+  @Test
+  fun initDisplayMetrics_preservesScaledDensityForFontScale() {
+    val originalMetrics = context.resources.displayMetrics
+    val customScaledDensity = originalMetrics.density * 0.85f // fontScale = 0.85
+
+    originalMetrics.scaledDensity = customScaledDensity
+
+    DisplayMetricsHolder.initDisplayMetrics(context)
+
+    val screenMetrics = DisplayMetricsHolder.getScreenDisplayMetrics()
+
+    assertThat(screenMetrics.scaledDensity).isEqualTo(customScaledDensity)
+  }
 }

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/uimanager/PixelUtilTest.kt

@@ -0,0 +1,194 @@
+/*
+ * 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.
+ */
+
+package com.facebook.react.uimanager
+
+import android.content.Context
+import android.content.res.Configuration
+import android.content.res.Resources
+import android.util.DisplayMetrics
+import com.facebook.testutils.shadows.ShadowNativeLoader
+import com.facebook.testutils.shadows.ShadowSoLoader
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.After
+import org.junit.Before
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mockito.kotlin.mock
+import org.mockito.kotlin.whenever
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import org.robolectric.annotation.Config
+import kotlin.math.abs
+
+@RunWith(RobolectricTestRunner::class)
+@Config(shadows = [ShadowSoLoader::class, ShadowNativeLoader::class])
+class PixelUtilTest {
+
+  private lateinit var context: Context
+
+  @Before
+  fun setUp() {
+    context = RuntimeEnvironment.getApplication()
+    DisplayMetricsHolder.setWindowDisplayMetrics(null)
+    DisplayMetricsHolder.setScreenDisplayMetrics(null)
+  }
+
+  @After
+  fun tearDown() {
+    DisplayMetricsHolder.setWindowDisplayMetrics(null)
+    DisplayMetricsHolder.setScreenDisplayMetrics(null)
+  }
+
+  @Test
+  fun toPixelFromSP_respectsFontScaleLessThanOne() {
+    // Setup display metrics with fontScale < 1.0
+    val displayMetrics = DisplayMetrics()
+    displayMetrics.density = 3.0f
+    displayMetrics.scaledDensity = 3.0f * 0.85f // fontScale = 0.85
+    displayMetrics.widthPixels = 1080
+    displayMetrics.heightPixels = 1920
+    displayMetrics.densityDpi = 480
+
+    DisplayMetricsHolder.setWindowDisplayMetrics(displayMetrics)
+    DisplayMetricsHolder.setScreenDisplayMetrics(displayMetrics)
+
+    // Test that toPixelFromSP respects fontScale < 1.0
+    val fontSize = 16f // 16sp
+    val result = PixelUtil.toPixelFromSP(fontSize)
+
+    // Expected: 16sp * 3.0 (density) * 0.85 (fontScale) = 40.8px
+    val expected = fontSize * displayMetrics.scaledDensity
+
+    assertThat(abs(result - expected)).isLessThan(0.1f)
+  }
+
+  @Test
+  fun toPixelFromSP_respectsFontScaleGreaterThanOne() {
+    // Setup display metrics with fontScale > 1.0
+    val displayMetrics = DisplayMetrics()
+    displayMetrics.density = 3.0f
+    displayMetrics.scaledDensity = 3.0f * 1.3f // fontScale = 1.3
+    displayMetrics.widthPixels = 1080
+    displayMetrics.heightPixels = 1920
+    displayMetrics.densityDpi = 480
+
+    DisplayMetricsHolder.setWindowDisplayMetrics(displayMetrics)
+    DisplayMetricsHolder.setScreenDisplayMetrics(displayMetrics)
+
+    // Test that toPixelFromSP respects fontScale > 1.0
+    val fontSize = 16f // 16sp
+    val result = PixelUtil.toPixelFromSP(fontSize)
+
+    // Expected: 16sp * 3.0 (density) * 1.3 (fontScale) = 62.4px
+    val expected = fontSize * displayMetrics.scaledDensity
+
+    assertThat(abs(result - expected)).isLessThan(0.1f)
+  }
+
+  @Test
+  fun toPixelFromSP_respectsMaxFontScale() {
+    // Setup display metrics with high fontScale
+    val displayMetrics = DisplayMetrics()
+    displayMetrics.density = 3.0f
+    displayMetrics.scaledDensity = 3.0f * 2.0f // fontScale = 2.0
+    displayMetrics.widthPixels = 1080
+    displayMetrics.heightPixels = 1920
+    displayMetrics.densityDpi = 480
+
+    DisplayMetricsHolder.setWindowDisplayMetrics(displayMetrics)
+    DisplayMetricsHolder.setScreenDisplayMetrics(displayMetrics)
+
+    // Test that maxFontScale limits the scaling
+    val fontSize = 16f // 16sp
+    val maxFontScale = 1.5f
+    val result = PixelUtil.toPixelFromSP(fontSize, maxFontScale)
+
+    // With fontScale = 2.0, scaledValue would be 16 * 3.0 * 2.0 = 96px
+    // But maxFontScale = 1.5 limits it to 16 * 3.0 * 1.5 = 72px
+    val expected = fontSize * displayMetrics.density * maxFontScale
+
+    assertThat(abs(result - expected)).isLessThan(0.1f)
+  }
+
+  @Test
+  fun toPixelFromSP_doesNotApplyMaxFontScaleWhenFontScaleIsLess() {
+    // Setup display metrics with low fontScale
+    val displayMetrics = DisplayMetrics()
+    displayMetrics.density = 3.0f
+    displayMetrics.scaledDensity = 3.0f * 0.8f // fontScale = 0.8
+    displayMetrics.widthPixels = 1080
+    displayMetrics.heightPixels = 1920
+    displayMetrics.densityDpi = 480
+
+    DisplayMetricsHolder.setWindowDisplayMetrics(displayMetrics)
+    DisplayMetricsHolder.setScreenDisplayMetrics(displayMetrics)
+
+    // Test that maxFontScale doesn't prevent scaling down
+    val fontSize = 16f // 16sp
+    val maxFontScale = 1.5f
+    val result = PixelUtil.toPixelFromSP(fontSize, maxFontScale)
+
+    // With fontScale = 0.8, scaledValue is 16 * 3.0 * 0.8 = 38.4px
+    // maxFontScale limit would be 16 * 3.0 * 1.5 = 72px
+    // min(38.4, 72) = 38.4px, so fontScale is respected
+    val expected = fontSize * displayMetrics.scaledDensity
+
+    assertThat(abs(result - expected)).isLessThan(0.1f)
+  }
+
+  @Test
+  fun toPixelFromDIP_convertsCorrectly() {
+    val displayMetrics = DisplayMetrics()
+    displayMetrics.density = 3.0f
+    displayMetrics.widthPixels = 1080
+    displayMetrics.heightPixels = 1920
+    displayMetrics.densityDpi = 480
+
+    DisplayMetricsHolder.setScreenDisplayMetrics(displayMetrics)
+
+    val dipValue = 16f
+    val result = PixelUtil.toPixelFromDIP(dipValue)
+
+    // Expected: 16dp * 3.0 (density) = 48px
+    val expected = dipValue * displayMetrics.density
+
+    assertThat(abs(result - expected)).isLessThan(0.1f)
+  }
+
+  @Test
+  fun initDisplayMetrics_preservesFontScale() {
+    // Create a context with custom configuration
+    val mockContext = mock<Context>()
+    val mockResources = mock<Resources>()
+    val configuration = Configuration()
+    configuration.fontScale = 0.85f
+
+    val displayMetrics = DisplayMetrics()
+    displayMetrics.density = 3.0f
+    displayMetrics.scaledDensity = 3.0f * 0.85f // fontScale = 0.85
+    displayMetrics.widthPixels = 1080
+    displayMetrics.heightPixels = 1920
+    displayMetrics.densityDpi = 480
+
+    whenever(mockContext.resources).thenReturn(mockResources)
+    whenever(mockResources.displayMetrics).thenReturn(displayMetrics)
+    whenever(mockResources.configuration).thenReturn(configuration)
+    whenever(mockContext.getSystemService(Context.WINDOW_SERVICE)).thenReturn(
+        context.getSystemService(Context.WINDOW_SERVICE)
+    )
+
+    // Initialize display metrics
+    DisplayMetricsHolder.initDisplayMetrics(mockContext)
+
+    val screenMetrics = DisplayMetricsHolder.getScreenDisplayMetrics()
+
+    // Verify that scaledDensity (which includes fontScale) is preserved
+    assertThat(abs(screenMetrics.scaledDensity - displayMetrics.scaledDensity))
+        .isLessThan(0.01f)
+  }
+}