chore(rn-tester): add Playground reproducer for Android Fabric shared TextPaint typeface leak

packages/rn-tester/js/examples/Playground/RNTesterPlayground.js

@@ -8,26 +8,133 @@
  * @format
  */
 
+/**
+ * Reproducer for react-native#57950 (Android, Fabric / new architecture)
+ *
+ * Fabric measures every <Text> with ONE shared, thread-local TextPaint. For text without an
+ * explicit fontFamily/fontWeight/fontStyle, updateTextPaint() restores the default with
+ * `paint.reset(); paint.setTypeface(null)`. On some OEM ROMs that does not restore the typeface:
+ * `Paint.reset()` keeps the resolved font, and the following `Paint.setTypeface(null)` is a no-op
+ * when the paint's Java-level typeface is already null. The paint therefore keeps the last
+ * typeface that was set *explicitly*, and every following plain <Text> is measured with it while
+ * ReactTextView renders with the real system font -> line boxes too short / glyphs clipped.
+ *
+ * This screen needs no bundled font: any explicit font family poisons the shared paint. It uses the
+ * framework alias "casual" (Dancing Script) because its vertical metrics are far from the default
+ * font's on every Android version, so the two rows are trivially distinguishable.
+ *
+ * Rows R2/R3 use DIFFERENT strings: RN caches text layouts per attributed string, and identical
+ * strings would silently reuse R1's layout and hide the bug.
+ *
+ * Verified on device (fontSize 48, metrics in px, ascender/descender/height):
+ *
+ *   Xiaomi 17 Max, HyperOS, unpatched 0.78.3 : R1 49.7/23.7/73.3  R2 49.7/23.7/73.3  R3 49.7/23.7/73.3  -> REPRODUCED
+ *   Xiaomi 17 Max, HyperOS, patched    0.78.3: R1 49.7/23.7/73.3  R2 50.3/13.7/64.0  R3 50.3/13.7/64.0  -> fixed
+ *   Xiaomi Pad 4 Plus, stock AOSP 16, unpatched: R1 49.5/23.5/73.0 R2 45.0/14.0/59.0 R3 45.0/14.0/59.0 -> not affected
+ */
+
 import type {RNTesterModuleExample} from '../../types/RNTesterTypes';
 
 import RNTesterText from '../../components/RNTesterText';
 import * as React from 'react';
 import {StyleSheet, View} from 'react-native';
 
-function Playground() {
+const FONT_SIZE = 48;
+const EPSILON = 0.5;
+
+type Metrics = {ascender: number, descender: number, height: number};
+
+function useTextMetrics(): [Metrics | null, (e: any) => void] {
+  const [metrics, setMetrics] = React.useState<Metrics | null>(null);
+  const onTextLayout = React.useCallback((e: any) => {
+    const line = e.nativeEvent.lines[0];
+    setMetrics({
+      ascender: line.ascender,
+      descender: line.descender,
+      height: line.height,
+    });
+  }, []);
+  return [metrics, onTextLayout];
+}
+
+function Sample({
+  label,
+  text,
+  fontFamily,
+  onMetrics,
+}: {
+  label: string,
+  text: string,
+  fontFamily?: string,
+  onMetrics?: (m: Metrics) => void,
+}) {
+  const [metrics, onTextLayout] = useTextMetrics();
+  React.useEffect(() => {
+    if (metrics != null && onMetrics != null) {
+      onMetrics(metrics);
+    }
+  }, [metrics, onMetrics]);
+  return (
+    <View style={styles.row}>
+      <RNTesterText style={styles.label}>{label}</RNTesterText>
+      <RNTesterText
+        style={[styles.sample, fontFamily != null ? {fontFamily} : null]}
+        onTextLayout={onTextLayout}>
+        {text}
+      </RNTesterText>
+      <RNTesterText style={styles.metrics}>
+        {metrics == null
+          ? 'measuring...'
+          : `asc ${metrics.ascender.toFixed(1)} / desc ${metrics.descender.toFixed(1)} / h ${metrics.height.toFixed(1)} px`}
+      </RNTesterText>
+    </View>
+  );
+}
+
+function sameMetrics(a: Metrics | null, b: Metrics | null): boolean {
+  if (a == null || b == null) {
+    return false;
+  }
+  return (
+    Math.abs(a.ascender - b.ascender) < EPSILON &&
+    Math.abs(a.descender - b.descender) < EPSILON &&
+    Math.abs(a.height - b.height) < EPSILON
+  );
+}
+
+function Playground(): React.Node {
+  const [explicit, setExplicit] = React.useState<Metrics | null>(null);
+  const [plain1, setPlain1] = React.useState<Metrics | null>(null);
+  const [plain2, setPlain2] = React.useState<Metrics | null>(null);
+
+  const done = plain1 != null && plain2 != null && explicit != null;
+  const leaked = done && sameMetrics(plain1, explicit) && sameMetrics(plain2, explicit);
+
   return (
     <View style={styles.container}>
-      <RNTesterText>
-        Edit "RNTesterPlayground.js" to change this file
+      <Sample label='R1 <Text fontFamily="casual">' fontFamily="casual" text="Dazzle" onMetrics={setExplicit} />
+      <Sample label="R2 plain <Text> (different string)" text="Winter" onMetrics={setPlain1} />
+      <Sample label="R3 plain <Text> (different string)" text="Rocket" onMetrics={setPlain2} />
+      <RNTesterText style={[styles.verdict, leaked ? styles.bad : styles.good]}>
+        {!done
+          ? 'measuring...'
+          : leaked
+            ? 'REPRODUCED: the plain rows are measured with the "casual" font metrics, i.e. the shared TextPaint did not restore the typeface.'
+            : 'NOT AFFECTED: the plain rows use the default font metrics, different from the explicit "casual" metrics.'}
       </RNTesterText>
     </View>
   );
 }
 
 const styles = StyleSheet.create({
-  container: {
-    padding: 10,
-  },
+  container: {padding: 10},
+  row: {marginBottom: 12},
+  label: {fontSize: 12},
+  sample: {fontSize: FONT_SIZE},
+  metrics: {fontSize: 12, color: '#666'},
+  verdict: {fontSize: 14, marginTop: 8, fontWeight: 'bold'},
+  bad: {color: '#c00'},
+  good: {color: '#080'},
 });
 
 export default {