fix(android): use the context's display for display metrics, not the device default

DisplayMetricsHolder.initDisplayMetrics populated screenDisplayMetrics by
calling getRealMetrics on WindowManager.defaultDisplay, which is always
the device's primary display regardless of which display the activity is
running on. On a secondary display (Samsung DeX, desktop mode, external
monitor, freeform window) that reports the primary display's density and
dimensions, so PixelUtil's dp <-> px conversion — and therefore Fabric's
layout — scales content for the wrong display. The visible region ends
up clipped to a fraction of the activity window and text renders at
sub-pixel positions.

Use Context.getDisplay() (API 30+) so the metrics come from the display
the context is actually associated with, falling back to defaultDisplay
on older API levels. Also pass the view's own context (not the
application context) from ReactRootView, so the context is associated
with the activity's display.

Fixes #56894 (also tracked in #55659).

Changelog: [ANDROID] [FIXED] - Display metrics now reflect the activity's display instead of the device's default display, fixing layout scaling on secondary displays / desktop mode / freeform windows.

Author: DouweBos

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactRootView.java

@@ -137,7 +137,7 @@ private void init() {
     setClipChildren(false);
 
     if (ReactNativeFeatureFlags.enableFontScaleChangesUpdatingLayout()) {
-      DisplayMetricsHolder.initDisplayMetrics(getContext().getApplicationContext());
+      DisplayMetricsHolder.initDisplayMetrics(getContext());
     }
   }
 
@@ -932,7 +932,7 @@ private class CustomGlobalLayoutListener implements ViewTreeObserver.OnGlobalLay
     private int mDeviceRotation = 0;
 
     /* package */ CustomGlobalLayoutListener() {
-      DisplayMetricsHolder.initDisplayMetricsIfNotInitialized(getContext().getApplicationContext());
+      DisplayMetricsHolder.initDisplayMetricsIfNotInitialized(getContext());
       mVisibleViewArea = new Rect();
     }
 
@@ -1007,7 +1007,7 @@ private void checkForDeviceOrientationChanges() {
         return;
       }
       mDeviceRotation = rotation;
-      DisplayMetricsHolder.initDisplayMetrics(getContext().getApplicationContext());
+      DisplayMetricsHolder.initDisplayMetrics(getContext());
       emitOrientationChanged(rotation);
     }
 

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

@@ -10,7 +10,9 @@ package com.facebook.react.uimanager
 import android.annotation.SuppressLint
 import android.app.Activity
 import android.content.Context
+import android.os.Build
 import android.util.DisplayMetrics
+import android.view.Display
 import android.view.WindowManager
 import androidx.core.view.ViewCompat
 import androidx.core.view.WindowInsetsCompat
@@ -54,9 +56,19 @@ public object DisplayMetricsHolder {
     val screenDisplayMetrics = DisplayMetrics()
     screenDisplayMetrics.setTo(displayMetrics)
     try {
-      val wm = context.getSystemService(Context.WINDOW_SERVICE) as WindowManager
+      // Use the display the context is associated with, not the device's
+      // default display. They differ when the activity is running on a
+      // secondary display (Samsung DeX, desktop mode, external monitor,
+      // freeform window) — using the default display there reports the
+      // primary display's density/dimensions and breaks layout scaling.
+      val display: Display? =
+          if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+            context.display
+          } else {
+            (context.getSystemService(Context.WINDOW_SERVICE) as WindowManager).defaultDisplay
+          }
       // getRealMetrics includes system decor (e.g. nav bar) excluded from resource metrics.
-      wm.defaultDisplay.getRealMetrics(screenDisplayMetrics)
+      display?.getRealMetrics(screenDisplayMetrics)
     } catch (_: Exception) {
       // Non-visual contexts (e.g. Application) may throw on API 30+.
       // Falls back to resource display metrics copied via setTo() above.