[web] Add new line break type (prohibited) (flutter/engine#22771)

engine/src/flutter/lib/web_ui/lib/src/engine/text/line_breaker.dart

@@ -10,6 +10,9 @@ enum LineBreakType {
   /// Indicates that a line break is possible but not mandatory.
   opportunity,
 
+  /// Indicates that a line break isn't possible.
+  prohibited,
+
   /// Indicates that this is a hard line break that can't be skipped.
   mandatory,
 
@@ -74,6 +77,9 @@ class LineBreakResult {
   /// to decide whether to take the line break or not.
   final LineBreakType type;
 
+  bool get isHard =>
+      type == LineBreakType.mandatory || type == LineBreakType.endOfText;
+
   @override
   int get hashCode => ui.hashValues(
         index,
@@ -160,7 +166,7 @@ bool _hasEastAsianWidthFWH(int charCode) {
 ///
 /// * https://www.unicode.org/reports/tr14/tr14-45.html#Algorithm
 /// * https://www.unicode.org/Public/11.0.0/ucd/LineBreak.txt
-LineBreakResult nextLineBreak(String text, int index) {
+LineBreakResult nextLineBreak(String text, int index, {int? maxEnd}) {
   int? codePoint = getCodePoint(text, index);
   LineCharProperty curr = lineLookup.findForChar(codePoint);
 
@@ -199,6 +205,15 @@ LineBreakResult nextLineBreak(String text, int index) {
   // Always break at the end of text.
   // LB3: ! eot
   while (index < text.length) {
+    if (index == maxEnd) {
+      return LineBreakResult(
+        index,
+        lastNonNewlineIndex,
+        lastNonSpaceIndex,
+        LineBreakType.prohibited,
+      );
+    }
+
     // Keep count of the RI (regional indicator) sequence.
     if (curr == LineCharProperty.RI) {
       regionalIndicatorCount++;

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

@@ -771,8 +771,6 @@ class LinesCalculator {
   /// This method should be called for every line break. As soon as it reaches
   /// the maximum number of lines required
   void update(LineBreakResult brk) {
-    final bool isHardBreak = brk.type == LineBreakType.mandatory ||
-        brk.type == LineBreakType.endOfText;
     final int chunkEnd = brk.index;
     final int chunkEndWithoutNewlines = brk.indexWithoutTrailingNewlines;
     final int chunkEndWithoutSpace = brk.indexWithoutTrailingSpaces;
@@ -855,7 +853,7 @@ class LinesCalculator {
       return;
     }
 
-    if (isHardBreak) {
+    if (brk.isHard) {
       _addLineBreak(brk);
     }
     _lastBreak = brk;
@@ -872,15 +870,13 @@ class LinesCalculator {
       lineWidth: lineWidth,
       maxWidth: _maxWidth,
     );
-    final bool isHardBreak = brk.type == LineBreakType.mandatory ||
-        brk.type == LineBreakType.endOfText;
 
     final EngineLineMetrics metrics = EngineLineMetrics.withText(
       _text!.substring(_lineStart, brk.indexWithoutTrailingNewlines),
       startIndex: _lineStart,
       endIndex: brk.index,
       endIndexWithoutNewlines: brk.indexWithoutTrailingNewlines,
-      hardBreak: isHardBreak,
+      hardBreak: brk.isHard,
       width: lineWidth,
       widthWithTrailingSpaces: lineWidthWithTrailingSpaces,
       left: alignOffset,
@@ -995,7 +991,7 @@ class MaxIntrinsicCalculator {
   /// intrinsic width calculated so far. When the whole text is consumed,
   /// [value] will contain the final maximum intrinsic width.
   void update(LineBreakResult brk) {
-    if (brk.type == LineBreakType.opportunity) {
+    if (!brk.isHard) {
       return;
     }
 

engine/src/flutter/lib/web_ui/test/text/line_breaker_test.dart

@@ -252,6 +252,14 @@ void testMain() {
                   '"$text"\n'
                   '\nExpected line break at {$lastLineBreak - $i} but found line break at {$lastLineBreak - ${result.index}}.',
             );
+
+            // Since this is a line break, passing a `maxEnd` that's greater
+            // should return the same line break.
+            final LineBreakResult maxEndResult =
+                nextLineBreak(text, lastLineBreak, maxEnd: i + 1);
+            expect(maxEndResult.index, i);
+            expect(maxEndResult.type, isNot(LineBreakType.prohibited));
+
             lastLineBreak = i;
           } else {
             // This isn't a line break opportunity so the line break should be
@@ -264,6 +272,13 @@ void testMain() {
                   '"$text"\n'
                   '\nUnexpected line break found at {$lastLineBreak - $i}.',
             );
+
+            // Since this isn't a line break, passing it as a `maxEnd` should
+            // return `maxEnd` as a prohibited line break type.
+            final LineBreakResult maxEndResult =
+                nextLineBreak(text, lastLineBreak, maxEnd: i);
+            expect(maxEndResult.index, i);
+            expect(maxEndResult.type, LineBreakType.prohibited);
           }
         }
       }