fix getFullHeightForCaret when strut is disabled. (#154039)

Fixes https://github.com/flutter/flutter/issues/150638

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

@@ -371,6 +371,7 @@ class _TextLayout {
 
     final double baseline = lineMetrics.baseline;
     final double dx;
+    final double height;
     late final ui.GlyphInfo? lastGlyph = _paragraph.getGlyphInfoAt(rawString.length - 1);
     // TODO(LongCatIsLooong): handle the case where maxLine is set to non-null
     // and the last line ends with trailing whitespaces.
@@ -381,13 +382,15 @@ class _TextLayout {
         TextDirection.ltr => glyphBounds.right,
         TextDirection.rtl => glyphBounds.left,
       };
+      height = glyphBounds.height;
     } else {
       dx = switch (writingDirection) {
         TextDirection.ltr => lineMetrics.left + lineMetrics.width,
         TextDirection.rtl => lineMetrics.left,
       };
+      height = lineMetrics.height;
     }
-    return _LineCaretMetrics(offset: Offset(dx, baseline), writingDirection: writingDirection);
+    return _LineCaretMetrics(offset: Offset(dx, baseline), writingDirection: writingDirection, height: height);
   }
 
   double _contentWidthFor(double minWidth, double maxWidth, TextWidthBasis widthBasis) {
@@ -501,8 +504,8 @@ class _TextPainterLayoutCacheWithOffset {
 /// The _CaretMetrics for carets located in a non-empty paragraph. Such carets
 /// are anchored to the trailing edge or the leading edge of a glyph, or a
 /// ligature component.
-final class _LineCaretMetrics {
-  const _LineCaretMetrics({required this.offset, required this.writingDirection});
+class _LineCaretMetrics {
+  const _LineCaretMetrics({required this.offset, required this.writingDirection, required this.height});
   /// The offset from the top left corner of the paragraph to the caret's top
   /// start location.
   final Offset offset;
@@ -512,10 +515,13 @@ final class _LineCaretMetrics {
   /// right of [offset].
   final TextDirection writingDirection;
 
+  /// The recommended height of the caret.
+  final double height;
+
   _LineCaretMetrics shift(Offset offset) {
     return offset == Offset.zero
       ? this
-      : _LineCaretMetrics(offset: offset + this.offset, writingDirection: writingDirection);
+      : _LineCaretMetrics(offset: offset + this.offset, writingDirection: writingDirection, height: height);
   }
 }
 
@@ -1367,6 +1373,16 @@ class TextPainter {
   ///
   /// Valid only after [layout] has been called.
   double getFullHeightForCaret(TextPosition position, Rect caretPrototype) {
+    // The if condition is derived from
+    // https://github.com/google/skia/blob/0086a17e0d4cc676cf88cae671ba5ee967eb7241/modules/skparagraph/src/TextLine.cpp#L1244-L1246
+    // which is set here:
+    // https://github.com/flutter/engine/blob/a821b8790c9fd0e095013cd5bd1f20273bc1ee47/third_party/txt/src/skia/paragraph_builder_skia.cc#L134
+    if (strutStyle == null || strutStyle == StrutStyle.disabled || strutStyle?.fontSize == 0.0) {
+      final double? heightFromCaretMetrics = _computeCaretMetrics(position)?.height;
+      if (heightFromCaretMetrics != null) {
+        return heightFromCaretMetrics;
+      }
+    }
     final TextBox textBox = _getOrCreateLayoutTemplate().getBoxesForRange(0, 1, boxHeightStyle: ui.BoxHeightStyle.strut).single;
     return textBox.toRect().height;
   }
@@ -1450,7 +1466,8 @@ class TextPainter {
 
     if (glyphInfo == null) {
       // If the glyph isn't laid out, then the position points to a character
-      // that is not laid out. Use the EOT caret.
+      // that is not laid out (the part of text is invisible due to maxLines or
+      // infinite paragraph x offset). Use the EOT caret.
       // TODO(LongCatIsLooong): assert when an invalid position is given.
       final ui.Paragraph template = _getOrCreateLayoutTemplate();
       assert(template.numberOfLines == 1);
@@ -1487,6 +1504,7 @@ class TextPainter {
       metrics = _LineCaretMetrics(
         offset: Offset(anchorToLeft ? box.left : box.right, box.top),
         writingDirection: box.direction,
+        height: box.bottom - box.top,
       );
     } else {
       // Fall back to glyphInfo. This should only happen when using the HTML renderer.
@@ -1499,6 +1517,7 @@ class TextPainter {
       metrics = _LineCaretMetrics(
         offset: Offset(dx, graphemeBounds.top),
         writingDirection: glyphInfo.writingDirection,
+        height: graphemeBounds.height,
       );
     }
 

packages/flutter/test/painting/text_painter_test.dart

@@ -857,6 +857,28 @@ void main() {
       expect(caretOffset.dy, 0.0);
       painter.dispose();
     });
+
+    test('caret height reflects run height if strut is disabled', () {
+      const TextSpan span = TextSpan(text: 'M', style: TextStyle(fontSize: 128), children: <InlineSpan>[
+        TextSpan(text: 'M', style: TextStyle(fontSize: 32)),
+        TextSpan(text: 'M', style: TextStyle(fontSize: 64)),
+      ]);
+      final TextPainter painter = TextPainter()
+        ..textDirection = TextDirection.ltr
+        ..text = span
+        ..layout();
+
+      expect(painter.getFullHeightForCaret(const TextPosition(offset: 0, affinity: ui.TextAffinity.upstream), Rect.zero), 128.0);
+      expect(painter.getFullHeightForCaret(const TextPosition(offset: 0), Rect.zero), 128.0);
+      expect(painter.getFullHeightForCaret(const TextPosition(offset: 1, affinity: ui.TextAffinity.upstream), Rect.zero), 128.0);
+      expect(painter.getFullHeightForCaret(const TextPosition(offset: 1), Rect.zero), 32.0);
+      expect(painter.getFullHeightForCaret(const TextPosition(offset: 2, affinity: ui.TextAffinity.upstream), Rect.zero), 32.0);
+      expect(painter.getFullHeightForCaret(const TextPosition(offset: 2), Rect.zero), 64.0);
+      expect(painter.getFullHeightForCaret(const TextPosition(offset: 3, affinity: ui.TextAffinity.upstream), Rect.zero), 64.0);
+      expect(painter.getFullHeightForCaret(const TextPosition(offset: 3), Rect.zero), 128.0);
+
+      painter.dispose();
+    });
   });
 
   test('TextPainter error test', () {