[web] Correct getPositionForOffset for multi-line paragraphs (#16206)

lib/web_ui/lib/src/engine/text/measurement.dart

@@ -373,7 +373,10 @@ class DomTextMeasurementService extends TextMeasurementService {
   @override
   ui.TextPosition getTextPositionForOffset(EngineParagraph paragraph,
       ui.ParagraphConstraints constraints, ui.Offset offset) {
-    assert(paragraph._plainText == null, 'should only be called for multispan');
+    assert(
+      paragraph._measurementResult.lines == null,
+      'should only be called when the faster lines-based approach is not possible',
+    );
 
     final ParagraphGeometricStyle style = paragraph._geometricStyle;
     final ParagraphRuler ruler =

lib/web_ui/lib/src/engine/text/paragraph.dart

@@ -378,17 +378,53 @@ class EngineParagraph implements ui.Paragraph {
 
   @override
   ui.TextPosition getPositionForOffset(ui.Offset offset) {
-    if (_plainText == null) {
+    final List<EngineLineMetrics> lines = _measurementResult.lines;
+    if (lines == null) {
       return getPositionForMultiSpanOffset(offset);
     }
+
+    final int lineNumber = offset.dy ~/ _measurementResult.lineHeight;
+
+    // [offset] is below all the lines.
+    if (lineNumber >= lines.length) {
+      return ui.TextPosition(
+        offset: _plainText.length,
+        affinity: ui.TextAffinity.upstream,
+      );
+    }
+
+    final EngineLineMetrics lineMetrics = lines[lineNumber];
+    final double lineLeft = lineMetrics.left;
+    final double lineRight = lineLeft + lineMetrics.width;
+
+    // [offset] is to the left of the line.
+    if (offset.dx <= lineLeft) {
+      return ui.TextPosition(
+        offset: lineMetrics.startIndex,
+        affinity: ui.TextAffinity.downstream,
+      );
+    }
+
+    // [offset] is to the right of the line.
+    if (offset.dx >= lineRight) {
+      return ui.TextPosition(
+        offset: lineMetrics.endIndex,
+        affinity: ui.TextAffinity.upstream,
+      );
+    }
+
+    // If we reach here, it means the [offset] is somewhere within the line. The
+    // code below will do a binary search to find where exactly the [offset]
+    // falls within the line.
+
     final double dx = offset.dx - _alignOffset;
     final TextMeasurementService instance = _measurementService;
 
-    int low = 0;
-    int high = _plainText.length;
+    int low = lineMetrics.startIndex;
+    int high = lineMetrics.endIndex;
     do {
       final int current = (low + high) ~/ 2;
-      final double width = instance.measureSubstringWidth(this, 0, current);
+      final double width = instance.measureSubstringWidth(this, lineMetrics.startIndex, current);
       if (width < dx) {
         low = current;
       } else if (width > dx) {
@@ -403,8 +439,8 @@ class EngineParagraph implements ui.Paragraph {
       return ui.TextPosition(offset: high, affinity: ui.TextAffinity.upstream);
     }
 
-    final double lowWidth = instance.measureSubstringWidth(this, 0, low);
-    final double highWidth = instance.measureSubstringWidth(this, 0, high);
+    final double lowWidth = instance.measureSubstringWidth(this, lineMetrics.startIndex, low);
+    final double highWidth = instance.measureSubstringWidth(this, lineMetrics.startIndex, high);
 
     if (dx - lowWidth < highWidth - dx) {
       // The offset is closer to the low index.

lib/web_ui/test/paragraph_test.dart

@@ -8,7 +8,7 @@ import 'package:ui/ui.dart';
 import 'package:test/test.dart';
 
 void testEachMeasurement(String description, VoidCallback body, {bool skip}) {
-  test(description, () async {
+  test('$description (dom measurement)', () async {
     try {
       TextMeasurementService.initialize(rulerCacheCapacity: 2);
       return body();
@@ -130,6 +130,154 @@ void main() async {
   }, // TODO(nurhan): https://github.com/flutter/flutter/issues/46638
       skip: (browserEngine == BrowserEngine.firefox));
 
+  testEachMeasurement('getPositionForOffset single-line', () {
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: 10,
+      textDirection: TextDirection.ltr,
+    ));
+    builder.addText('abcd efg');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(const ParagraphConstraints(width: 1000));
+
+    // At the beginning of the line.
+    expect(
+      paragraph.getPositionForOffset(Offset(0, 5)),
+      TextPosition(offset: 0, affinity: TextAffinity.downstream),
+    );
+    // Below the line.
+    expect(
+      paragraph.getPositionForOffset(Offset(0, 12)),
+      TextPosition(offset: 8, affinity: TextAffinity.upstream),
+    );
+    // Above the line.
+    expect(
+      paragraph.getPositionForOffset(Offset(0, -5)),
+      TextPosition(offset: 0, affinity: TextAffinity.downstream),
+    );
+    // At the end of the line.
+    expect(
+      paragraph.getPositionForOffset(Offset(80, 5)),
+      TextPosition(offset: 8, affinity: TextAffinity.upstream),
+    );
+    // On the left side of "b".
+    expect(
+      paragraph.getPositionForOffset(Offset(14, 5)),
+      TextPosition(offset: 1, affinity: TextAffinity.downstream),
+    );
+    // On the right side of "b".
+    expect(
+      paragraph.getPositionForOffset(Offset(16, 5)),
+      TextPosition(offset: 2, affinity: TextAffinity.upstream),
+    );
+  });
+
+  test('getPositionForOffset multi-line', () {
+    // [Paragraph.getPositionForOffset] for multi-line text doesn't work well
+    // with dom-based measurement.
+    TextMeasurementService.enableExperimentalCanvasImplementation = true;
+    TextMeasurementService.initialize(rulerCacheCapacity: 2);
+
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: 10,
+      textDirection: TextDirection.ltr,
+    ));
+    builder.addText('abcd\n');
+    builder.addText('abcdefg\n');
+    builder.addText('ab');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(const ParagraphConstraints(width: 1000));
+
+    // First line: "abcd\n"
+
+    // At the beginning of the first line.
+    expect(
+      paragraph.getPositionForOffset(Offset(0, 5)),
+      TextPosition(offset: 0, affinity: TextAffinity.downstream),
+    );
+    // Above the first line.
+    expect(
+      paragraph.getPositionForOffset(Offset(0, -5)),
+      TextPosition(offset: 0, affinity: TextAffinity.downstream),
+    );
+    // At the end of the first line.
+    expect(
+      paragraph.getPositionForOffset(Offset(50, 5)),
+      TextPosition(offset: 5, affinity: TextAffinity.upstream),
+    );
+    // On the left side of "b" in the first line.
+    expect(
+      paragraph.getPositionForOffset(Offset(14, 5)),
+      TextPosition(offset: 1, affinity: TextAffinity.downstream),
+    );
+    // On the right side of "b" in the first line.
+    expect(
+      paragraph.getPositionForOffset(Offset(16, 5)),
+      TextPosition(offset: 2, affinity: TextAffinity.upstream),
+    );
+
+
+    // Second line: "abcdefg\n"
+
+    // At the beginning of the second line.
+    expect(
+      paragraph.getPositionForOffset(Offset(0, 15)),
+      TextPosition(offset: 5, affinity: TextAffinity.downstream),
+    );
+    // At the end of the second line.
+    expect(
+      paragraph.getPositionForOffset(Offset(100, 15)),
+      TextPosition(offset: 13, affinity: TextAffinity.upstream),
+    );
+    // On the left side of "e" in the second line.
+    expect(
+      paragraph.getPositionForOffset(Offset(44, 15)),
+      TextPosition(offset: 9, affinity: TextAffinity.downstream),
+    );
+    // On the right side of "e" in the second line.
+    expect(
+      paragraph.getPositionForOffset(Offset(46, 15)),
+      TextPosition(offset: 10, affinity: TextAffinity.upstream),
+    );
+
+
+    // Last (third) line: "ab"
+
+    // At the beginning of the last line.
+    expect(
+      paragraph.getPositionForOffset(Offset(0, 25)),
+      TextPosition(offset: 13, affinity: TextAffinity.downstream),
+    );
+    // At the end of the last line.
+    expect(
+      paragraph.getPositionForOffset(Offset(40, 25)),
+      TextPosition(offset: 15, affinity: TextAffinity.upstream),
+    );
+    // Below the last line.
+    expect(
+      paragraph.getPositionForOffset(Offset(0, 32)),
+      TextPosition(offset: 15, affinity: TextAffinity.upstream),
+    );
+    // On the left side of "b" in the last line.
+    expect(
+      paragraph.getPositionForOffset(Offset(12, 25)),
+      TextPosition(offset: 14, affinity: TextAffinity.downstream),
+    );
+    // On the right side of "a" in the last line.
+    expect(
+      paragraph.getPositionForOffset(Offset(9, 25)),
+      TextPosition(offset: 14, affinity: TextAffinity.upstream),
+    );
+
+    TextMeasurementService.clearCache();
+    TextMeasurementService.enableExperimentalCanvasImplementation = false;
+  });
+
   testEachMeasurement('getBoxesForRange returns a box', () {
     final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
       fontFamily: 'Ahem',