Improved handling of user input around embeds

packages/notus/lib/src/document.dart

 
     if (_delta != _root.toDelta()) {
       throw new StateError('Compose produced inconsistent results. '
-          'This is likely due to a bug in the library.');
+          'This is likely due to a bug in the library. Tried to compose change $change from $source.');
     }
     _controller.add(new NotusChange(before, change, source));
   }

packages/notus/lib/src/heuristics/delete_rules.dart

   @override
   Delta apply(Delta document, int index, int length) {
     DeltaIterator iter = new DeltaIterator(document);
+
     // First, check if line-break deleted after an embed.
     Operation op = iter.skip(index);
     int indexDelta = 0;
     int lengthDelta = 0;
     int remaining = length;
     bool foundEmbed = false;
+    bool hasLineBreakBefore = false;
     if (op != null && op.data.endsWith(kZeroWidthSpace)) {
       foundEmbed = true;
       Operation candidate = iter.next(1);
           lengthDelta += 1;
         }
       }
+    } else {
+      // If op is `null` it's a beginning of the doc, e.g. implicit line break.
+      hasLineBreakBefore = op == null || op.data.endsWith('\n');
     }
 
     // Second, check if line-break deleted before an embed.
     op = iter.skip(remaining);
     if (op != null && op.data.endsWith('\n')) {
       final candidate = iter.next(1);
-      if (candidate.data == kZeroWidthSpace) {
+      // If there is a line-break before deleted range we allow the operation
+      // since it results in a correctly formatted line with single embed in it.
+      if (candidate.data == kZeroWidthSpace && !hasLineBreakBefore) {
         foundEmbed = true;
         lengthDelta -= 1;
       }
     }
+
     if (foundEmbed) {
       return new Delta()
         ..retain(index + indexDelta)

packages/notus/test/heuristics/delete_rules_test.dart

         ..delete(1);
       expect(actual, expected);
     });
+
+    test('allows deleting empty line(s) before embed', () {
+      final hr = NotusAttribute.embed.horizontalRule;
+      final doc = new Delta()
+        ..insert('Document\n')
+        ..insert('\n')
+        ..insert('\n')
+        ..insert(kZeroWidthSpace, hr.toJson())
+        ..insert('\n')
+        ..insert('Text')
+        ..insert('\n');
+      final actual = rule.apply(doc, 11, 1);
+      expect(actual, isNull);
+    });
   });
 }

packages/zefyr/example/pubspec.yaml

   zefyr:
     path: ../
 
+dependency_overrides:
+  notus:
+    path: ../../notus
+
 dev_dependencies:
   flutter_test:
     sdk: flutter

packages/zefyr/lib/util.dart

     } else if (userOp.isDelete && actualOp.isRetain) {
       diff += userOp.length;
     } else if (userOp.isRetain && actualOp.isInsert) {
+      if (actualOp.data.startsWith('\n') ) {
+        // At this point user input reached its end (retain). If a heuristic
+        // rule inserts a new line we should keep cursor on it's original position.
+        continue;
+      }
       diff += actualOp.length;
     } else {
       // TODO: this likely needs to cover more edge cases.