Fix off by one error in TextPainter. (#19429)

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

@@ -426,7 +426,7 @@ class TextPainter {
   }
 
   Offset _getOffsetFromDownstream(int offset, Rect caretPrototype) {
-    final int nextCodeUnit = _text.codeUnitAt(offset);
+    final int nextCodeUnit = _text.codeUnitAt(offset - 1);
     if (nextCodeUnit == null)
       return null;
     final int nextRuneOffset = _isUtf16Surrogate(nextCodeUnit) ? offset + 2 : offset + 1;

packages/flutter/test/painting/text_painter_test.dart

@@ -179,4 +179,117 @@ void main() {
     expect(painter.minIntrinsicWidth, 90.0);
     expect(painter.maxIntrinsicWidth, 180.0);
   }, skip: true); // https://github.com/flutter/flutter/issues/13512
+
+  test('TextPainter handles newlines properly', () {
+    final TextPainter painter = new TextPainter()
+      ..textDirection = TextDirection.ltr;
+
+    String text = 'aaa';
+    painter.text = new TextSpan(text: text);
+    painter.layout();
+
+    Offset caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 0), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    caretOffset = painter.getOffsetForCaret(new ui.TextPosition(offset: text.length), ui.Rect.zero);
+    expect(caretOffset.dx, painter.width);
+    expect(caretOffset.dy, closeTo(0.0, 0.0001));
+
+    // Check that getOffsetForCaret handles a trailing newline when affinity is downstream.
+    text = 'aaa\n';
+    painter.text = new TextSpan(text: text);
+    painter.layout();
+    caretOffset = painter.getOffsetForCaret(new ui.TextPosition(offset: text.length), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(14.0, 0.0001));
+
+    // Check that getOffsetForCaret handles a trailing newline when affinity is upstream.
+    text = 'aaa\n';
+    painter.text = new TextSpan(text: text);
+    painter.layout();
+    caretOffset = painter.getOffsetForCaret(new ui.TextPosition(offset: text.length, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, painter.width);
+    expect(caretOffset.dy, closeTo(0.0, 0.0001));
+
+    // Correctly moves through second line with downstream affinity.
+    text = 'aaa\naaa';
+    painter.text = new TextSpan(text: text);
+    painter.layout();
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 4), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(14.0, 0.0001));
+
+    // Correctly moves through second line with upstream affinity.
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 4, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(42.0, 0.0001));
+    expect(caretOffset.dy, closeTo(0.0, 0.0001));
+
+    // Correctly handles multiple trailing newlines.
+    text = 'aaa\n\n\n';
+    painter.text = new TextSpan(text: text);
+    painter.layout();
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 4), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(14.0, 0.001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 5), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(28.0, 0.001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 6), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(42.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 6, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(28.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 5, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(14.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 4, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(42.0, 0.0001));
+    expect(caretOffset.dy, closeTo(0.0, 0.0001));
+    
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 3, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(42.0, 0.0001));
+    expect(caretOffset.dy, closeTo(0.0, 0.0001));
+
+    // Correctly handles multiple leading newlines
+    text = '\n\n\naaa';
+    painter.text = new TextSpan(text: text);
+    painter.layout();
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 3), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(42.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 2), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(28.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 1), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy,closeTo(14.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 0), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(0.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 0, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(0.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 1, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(0.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 2, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(14.0, 0.0001));
+
+    caretOffset = painter.getOffsetForCaret(const ui.TextPosition(offset: 3, affinity: TextAffinity.upstream), ui.Rect.zero);
+    expect(caretOffset.dx, closeTo(0.0, 0.0001));
+    expect(caretOffset.dy, closeTo(28.0, 0.0001));
+  });
 }