Add missing TextPainter.strutStyle to paragraph style (#143771)

Fixes an issue where if the `TextSpan` doesn't have a text style, the specified strutStyle is not applied.

Additionally, adds a migration flag for https://github.com/flutter/engine/pull/50385, for internal golden changes. It's only going to exist for a week.

packages/flutter/lib/src/painting/text_painter.dart

@@ -24,6 +24,7 @@ import 'placeholder_span.dart';
 import 'strut_style.dart';
 import 'text_scaler.dart';
 import 'text_span.dart';
+import 'text_style.dart';
 
 export 'dart:ui' show LineMetrics;
 export 'package:flutter/services.dart' show TextRange, TextSelection;
@@ -632,6 +633,12 @@ class TextPainter {
     }
   }
 
+  @Deprecated(  // flutter_ignore: deprecation_syntax (see analyze.dart)
+    'The disableStrutHalfLeading flag is for internal migration purposes only and should not be used.'
+  )
+  /// Migration only flag, do not use.
+  static bool disableStrutHalfLeading = true;
+
   // Whether textWidthBasis has changed after the most recent `layout` call.
   bool _debugNeedsRelayout = true;
   // The result of the most recent `layout` call.
@@ -968,7 +975,27 @@ class TextPainter {
     // The defaultTextDirection argument is used for preferredLineHeight in case
     // textDirection hasn't yet been set.
     assert(textDirection != null || defaultTextDirection != null, 'TextPainter.textDirection must be set to a non-null value before using the TextPainter.');
-    return _text!.style?.getParagraphStyle(
+    final TextStyle baseStyle = _text?.style ?? const TextStyle();
+    final StrutStyle? strutStyle = _strutStyle;
+
+    final bool applyMigration = !kIsWeb && TextPainter.disableStrutHalfLeading
+                             && strutStyle != null && (strutStyle.forceStrutHeight ?? false)
+                             && strutStyle.leadingDistribution == TextLeadingDistribution.even;
+    final StrutStyle? strutStyleForMigration = !applyMigration
+      ? strutStyle
+      : StrutStyle(
+        fontFamily: strutStyle.fontFamily,
+        fontFamilyFallback: strutStyle.fontFamilyFallback,
+        fontSize: strutStyle.fontSize,
+        height: strutStyle.height,
+        leadingDistribution: TextLeadingDistribution.proportional,
+        leading: strutStyle.leading,
+        fontWeight: strutStyle.fontWeight,
+        fontStyle: strutStyle.fontStyle,
+        forceStrutHeight: strutStyle.forceStrutHeight,
+      );
+
+    return baseStyle.getParagraphStyle(
       textAlign: textAlign,
       textDirection: textDirection ?? defaultTextDirection,
       textScaler: textScaler,
@@ -976,18 +1003,7 @@ class TextPainter {
       textHeightBehavior: _textHeightBehavior,
       ellipsis: _ellipsis,
       locale: _locale,
-      strutStyle: _strutStyle,
-    ) ?? ui.ParagraphStyle(
-      textAlign: textAlign,
-      textDirection: textDirection ?? defaultTextDirection,
-      // Use the default font size to multiply by as RichText does not
-      // perform inheriting [TextStyle]s and would otherwise
-      // fail to apply textScaler.
-      fontSize: textScaler.scale(kDefaultFontSize),
-      maxLines: maxLines,
-      textHeightBehavior: _textHeightBehavior,
-      ellipsis: ellipsis,
-      locale: locale,
+      strutStyle: strutStyleForMigration,
     );
   }
 

packages/flutter/lib/src/painting/text_style.dart

@@ -1388,6 +1388,7 @@ class TextStyle with Diagnosticable {
         },
         height: strutStyle.height,
         leading: strutStyle.leading,
+        leadingDistribution: strutStyle.leadingDistribution,
         fontWeight: strutStyle.fontWeight,
         fontStyle: strutStyle.fontStyle,
         forceStrutHeight: strutStyle.forceStrutHeight,

packages/flutter/test/painting/text_painter_test.dart

@@ -1546,6 +1546,45 @@ void main() {
     }
   }, skip: kIsWeb && !isCanvasKit); // [intended] Browsers seem to always round font/glyph metrics.
 
+  group('strut style', () {
+    test('strut style applies when the span has no style', () {
+      const StrutStyle strut = StrutStyle(height: 10, fontSize: 10);
+      final TextPainter painter = TextPainter(
+        textDirection: TextDirection.ltr,
+        text: const TextSpan(),
+        strutStyle: strut,
+      )..layout();
+      expect(painter.height, 100);
+    });
+
+    test('strut style leading is a fontSize multiplier', () {
+      const StrutStyle strut = StrutStyle(height: 10, fontSize: 10, leading: 2);
+      final TextPainter painter = TextPainter(
+        textDirection: TextDirection.ltr,
+        text: const TextSpan(),
+        strutStyle: strut,
+      )..layout();
+      expect(painter.height, 100 + 20);
+      // Top leading + scaled ascent.
+      expect(painter.computeDistanceToActualBaseline(TextBaseline.alphabetic), 10 + 10 * 7.5);
+    });
+
+    test('force strut height', () {
+      const StrutStyle strut = StrutStyle(height: 10, fontSize: 10, forceStrutHeight: true);
+      final TextPainter painter = TextPainter(
+        textDirection: TextDirection.ltr,
+        text: const TextSpan(text: 'A', style: TextStyle(fontSize: 20)),
+        strutStyle: strut,
+      )..layout();
+      expect(painter.height, 100);
+      const double baseline = 75;
+      expect(
+        painter.getBoxesForSelection(const TextSelection(baseOffset: 0, extentOffset: 1)),
+        const <ui.TextBox>[TextBox.fromLTRBD(0, baseline - 15, 20, baseline + 5, TextDirection.ltr)],
+      );
+    });
+  }, skip: kIsWeb && !isCanvasKit); // [intended] strut spport for HTML renderer https://github.com/flutter/flutter/issues/32243.
+
   test('TextPainter dispatches memory events', () async {
     await expectLater(
       await memoryEvents(() => TextPainter().dispose(), TextPainter),