Fix getLineBoundary for TextAffinity (#28008)

Fix a bug in a calculation when selection is at a wordwrap

Author: justinmc

lib/ui/text.dart

@@ -3253,7 +3253,25 @@ class Paragraph extends NativeFieldWrapperClass1 {
   /// metrics, so use it sparingly.
   TextRange getLineBoundary(TextPosition position) {
     final List<int> boundary = _getLineBoundary(position.offset);
-    return TextRange(start: boundary[0], end: boundary[1]);
+    final TextRange line = TextRange(start: boundary[0], end: boundary[1]);
+
+    final List<int> nextBoundary = _getLineBoundary(position.offset + 1);
+    final TextRange nextLine = TextRange(start: nextBoundary[0], end: nextBoundary[1]);
+    // If there is no next line, because we're at the end of the field, return
+    // line.
+    if (!nextLine.isValid) {
+      return line;
+    }
+
+    // _getLineBoundary only considers the offset and assumes that the
+    // TextAffinity is upstream. In the case that TextPosition is just after a
+    // wordwrap (downstream), we need to return the line for the next offset.
+    if (position.affinity == TextAffinity.downstream && line != nextLine
+        && position.offset == line.end && line.end == nextLine.start) {
+      final List<int> nextBoundary = _getLineBoundary(position.offset + 1);
+      return TextRange(start: nextBoundary[0], end: nextBoundary[1]);
+    }
+    return line;
   }
   List<int> _getLineBoundary(int offset) native 'Paragraph_getLineBoundary';
 

testing/dart/paragraph_test.dart

@@ -59,4 +59,151 @@ void main() {
       );
     }
   });
+
+  test('getLineBoundary', () {
+    const double fontSize = 10.0;
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: fontSize,
+    ));
+    builder.addText('Test Ahem');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(ParagraphConstraints(width: fontSize * 5.0));
+
+    // Wraps to two lines.
+    expect(paragraph.height, closeTo(fontSize * 2.0, 0.001));
+
+    final TextPosition wrapPositionDown = TextPosition(
+      offset: 5,
+      affinity: TextAffinity.downstream,
+    );
+    TextRange line = paragraph.getLineBoundary(wrapPositionDown);
+    expect(line.start, 5);
+    expect(line.end, 9);
+
+    final TextPosition wrapPositionUp = TextPosition(
+      offset: 5,
+      affinity: TextAffinity.upstream,
+    );
+    line = paragraph.getLineBoundary(wrapPositionUp);
+    expect(line.start, 0);
+    expect(line.end, 5);
+
+    final TextPosition wrapPositionStart = TextPosition(
+      offset: 0,
+      affinity: TextAffinity.downstream,
+    );
+    line = paragraph.getLineBoundary(wrapPositionStart);
+    expect(line.start, 0);
+    expect(line.end, 5);
+
+    final TextPosition wrapPositionEnd = TextPosition(
+      offset: 9,
+      affinity: TextAffinity.downstream,
+    );
+    line = paragraph.getLineBoundary(wrapPositionEnd);
+    expect(line.start, 5);
+    expect(line.end, 9);
+  });
+
+  test('getLineBoundary RTL', () {
+    const double fontSize = 10.0;
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: fontSize,
+      textDirection: TextDirection.rtl,
+    ));
+    builder.addText('القاهرةالقاهرة');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(ParagraphConstraints(width: fontSize * 5.0));
+
+    // Wraps to three lines.
+    expect(paragraph.height, closeTo(fontSize * 3.0, 0.001));
+
+    final TextPosition wrapPositionDown = TextPosition(
+      offset: 5,
+      affinity: TextAffinity.downstream,
+    );
+    TextRange line = paragraph.getLineBoundary(wrapPositionDown);
+    expect(line.start, 5);
+    expect(line.end, 10);
+
+    final TextPosition wrapPositionUp = TextPosition(
+      offset: 5,
+      affinity: TextAffinity.upstream,
+    );
+    line = paragraph.getLineBoundary(wrapPositionUp);
+    expect(line.start, 0);
+    expect(line.end, 5);
+
+    final TextPosition wrapPositionStart = TextPosition(
+      offset: 0,
+      affinity: TextAffinity.downstream,
+    );
+    line = paragraph.getLineBoundary(wrapPositionStart);
+    expect(line.start, 0);
+    expect(line.end, 5);
+
+    final TextPosition wrapPositionEnd = TextPosition(
+      offset: 9,
+      affinity: TextAffinity.downstream,
+    );
+    line = paragraph.getLineBoundary(wrapPositionEnd);
+    expect(line.start, 5);
+    expect(line.end, 10);
+  });
+
+  test('getLineBoundary empty line', () {
+    const double fontSize = 10.0;
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: fontSize,
+      textDirection: TextDirection.rtl,
+    ));
+    builder.addText('Test\n\nAhem');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(ParagraphConstraints(width: fontSize * 5.0));
+
+    // Three lines due to line breaks, with the middle line being empty.
+    expect(paragraph.height, closeTo(fontSize * 3.0, 0.001));
+
+    final TextPosition emptyLinePosition = TextPosition(
+      offset: 5,
+      affinity: TextAffinity.downstream,
+    );
+    TextRange line = paragraph.getLineBoundary(emptyLinePosition);
+    expect(line.start, 5);
+    expect(line.end, 5);
+
+    // Since these are hard newlines, TextAffinity has no effect here.
+    final TextPosition emptyLinePositionUpstream = TextPosition(
+      offset: 5,
+      affinity: TextAffinity.upstream,
+    );
+    line = paragraph.getLineBoundary(emptyLinePositionUpstream);
+    expect(line.start, 5);
+    expect(line.end, 5);
+
+    final TextPosition endOfFirstLinePosition = TextPosition(
+      offset: 4,
+      affinity: TextAffinity.downstream,
+    );
+    line = paragraph.getLineBoundary(endOfFirstLinePosition);
+    expect(line.start, 0);
+    expect(line.end, 4);
+
+    final TextPosition startOfLastLinePosition = TextPosition(
+      offset: 6,
+      affinity: TextAffinity.downstream,
+    );
+    line = paragraph.getLineBoundary(startOfLastLinePosition);
+    expect(line.start, 6);
+    expect(line.end, 10);
+  });
 }