[web] Fix edge cases in Paragraph.getPositionForOffset to match Flutter

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

@@ -429,6 +429,8 @@ class DomTextMeasurementService extends TextMeasurementService {
           text,
           startIndex: 0,
           endIndex: text.length,
+          endIndexWithoutNewlines:
+              _excludeTrailing(text, 0, text.length, _newlinePredicate),
           hardBreak: true,
           width: lineWidth,
           left: alignOffset,
@@ -796,6 +798,8 @@ class LinesCalculator {
           _text.substring(_lineStart, breakingPoint) + _style.ellipsis,
           startIndex: _lineStart,
           endIndex: chunkEnd,
+          endIndexWithoutNewlines:
+              _excludeTrailing(_text, _chunkStart, chunkEnd, _newlinePredicate),
           hardBreak: false,
           width: widthOfResultingLine,
           left: alignOffset,
@@ -861,6 +865,7 @@ class LinesCalculator {
       _text.substring(_lineStart, endWithoutNewlines),
       startIndex: _lineStart,
       endIndex: lineEnd,
+      endIndexWithoutNewlines: endWithoutNewlines,
       hardBreak: isHardBreak,
       width: lineWidth,
       left: alignOffset,

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

@@ -17,12 +17,14 @@ class EngineLineMetrics implements ui.LineMetrics {
     this.lineNumber,
   })  : text = null,
         startIndex = -1,
-        endIndex = -1;
+        endIndex = -1,
+        endIndexWithoutNewlines = -1;
 
   EngineLineMetrics.withText(
     this.text, {
     @required this.startIndex,
     @required this.endIndex,
+    @required this.endIndexWithoutNewlines,
     @required this.hardBreak,
     this.ascent,
     this.descent,
@@ -33,6 +35,9 @@ class EngineLineMetrics implements ui.LineMetrics {
     this.baseline,
     @required this.lineNumber,
   })  : assert(text != null),
+        assert(startIndex != null),
+        assert(endIndex != null),
+        assert(endIndexWithoutNewlines != null),
         assert(hardBreak != null),
         assert(width != null),
         assert(left != null),
@@ -50,6 +55,10 @@ class EngineLineMetrics implements ui.LineMetrics {
   /// the text and doesn't stop at the overflow cutoff.
   final int endIndex;
 
+  /// The index (exclusive) in the text where this line ends, ignoring newline
+  /// characters.
+  final int endIndexWithoutNewlines;
+
   @override
   final bool hardBreak;
 
@@ -416,7 +425,7 @@ class EngineParagraph implements ui.Paragraph {
     // [offset] is to the right of the line.
     if (offset.dx >= lineRight) {
       return ui.TextPosition(
-        offset: lineMetrics.endIndex,
+        offset: lineMetrics.endIndexWithoutNewlines,
         affinity: ui.TextAffinity.upstream,
       );
     }
@@ -429,7 +438,7 @@ class EngineParagraph implements ui.Paragraph {
     final TextMeasurementService instance = _measurementService;
 
     int low = lineMetrics.startIndex;
-    int high = lineMetrics.endIndex;
+    int high = lineMetrics.endIndexWithoutNewlines;
     do {
       final int current = (low + high) ~/ 2;
       final double width = instance.measureSubstringWidth(this, lineMetrics.startIndex, current);

lib/web_ui/test/paragraph_test.dart

@@ -191,7 +191,7 @@ void main() async {
     builder.addText('abcdefg\n');
     builder.addText('ab');
     final Paragraph paragraph = builder.build();
-    paragraph.layout(const ParagraphConstraints(width: 1000));
+    paragraph.layout(const ParagraphConstraints(width: 100));
 
     // First line: "abcd\n"
 
@@ -208,7 +208,7 @@ void main() async {
     // At the end of the first line.
     expect(
       paragraph.getPositionForOffset(Offset(50, 5)),
-      TextPosition(offset: 5, affinity: TextAffinity.upstream),
+      TextPosition(offset: 4, affinity: TextAffinity.upstream),
     );
     // On the left side of "b" in the first line.
     expect(
@@ -232,7 +232,7 @@ void main() async {
     // At the end of the second line.
     expect(
       paragraph.getPositionForOffset(Offset(100, 15)),
-      TextPosition(offset: 13, affinity: TextAffinity.upstream),
+      TextPosition(offset: 12, affinity: TextAffinity.upstream),
     );
     // On the left side of "e" in the second line.
     expect(
@@ -255,7 +255,7 @@ void main() async {
     );
     // At the end of the last line.
     expect(
-      paragraph.getPositionForOffset(Offset(40, 25)),
+      paragraph.getPositionForOffset(Offset(100, 25)),
       TextPosition(offset: 15, affinity: TextAffinity.upstream),
     );
     // Below the last line.