Preserve line styles on new line insertion

Fixes #158

packages/parchment/lib/src/heuristics.dart

@@ -34,11 +34,13 @@ class ParchmentHeuristics {
       MarkdownBlockShortcutsInsertRule(),
       // Lines
       PreserveLineStyleOnSplitRule(),
-      ResetLineFormatOnNewLineRule(),
+      ResetHeadingFormatOnNewLineRule(),
       AutoTextDirectionRule(),
       // Inlines
       AutoFormatLinksRule(),
       PreserveInlineStylesRule(),
+      // Catch new line
+      PreserveLineStyleOnNewLineInsertRule(),
       // Catch-all
       CatchAllInsertRule(),
     ],

packages/parchment/lib/src/heuristics/insert_rules.dart

@@ -64,6 +64,25 @@ class CatchAllInsertRule extends InsertRule {
   }
 }
 
+/// Fallback rule for new line which retains line styles
+class PreserveLineStyleOnNewLineInsertRule extends InsertRule {
+  const PreserveLineStyleOnNewLineInsertRule();
+
+  @override
+  Delta? apply(Delta document, int index, Object data) {
+    if (!isOnlyNewLines(data)) return null;
+
+    final iter = DeltaIterator(document);
+    iter.skip(index);
+    final nextNewline = _findNextNewline(iter);
+    if (nextNewline.isEmpty) return null;
+
+    return Delta()
+      ..retain(index)
+      ..insert(data, nextNewline.op?.attributes);
+  }
+}
+
 /// Preserves line format when user splits the line into two.
 ///
 /// This rule ignores scenarios when the line is split on its edge, meaning
@@ -113,13 +132,13 @@ class PreserveLineStyleOnSplitRule extends InsertRule {
   }
 }
 
-/// Resets format for a newly inserted line when insert occurred at the end
-/// of a line (right before a newline).
+/// Resets heading format for a newly inserted line when insert occurred at
+/// the end of a line (right before a newline).
 ///
 /// This handles scenarios when a new line is added when at the end of a
 /// heading line. The newly added line should be a regular paragraph.
-class ResetLineFormatOnNewLineRule extends InsertRule {
-  const ResetLineFormatOnNewLineRule();
+class ResetHeadingFormatOnNewLineRule extends InsertRule {
+  const ResetHeadingFormatOnNewLineRule();
 
   @override
   Delta? apply(Delta document, int index, Object data) {
@@ -144,8 +163,7 @@ class ResetLineFormatOnNewLineRule extends InsertRule {
         return Delta()
           ..retain(index)
           ..insert('\n', target.attributes)
-          ..retain(1, resetStyle)
-          ..trim();
+          ..retain(1, resetStyle);
       }
     }
     return null;

packages/parchment/lib/src/heuristics/utils.dart

@@ -43,3 +43,8 @@ bool isBlockEmbed(Object data) {
   }
   return false;
 }
+
+bool isOnlyNewLines(Object data) {
+  if (data is! String) return false;
+  return data.replaceAll('\n', '').isEmpty;
+}

packages/parchment/test/heuristics/insert_rules_test.dart

@@ -22,6 +22,27 @@ void main() {
     });
   });
 
+  group('$PreserveLineStyleOnNewLineInsertRule', () {
+    final rule = PreserveLineStyleOnNewLineInsertRule();
+
+    test('data is not new line only', () {
+      final doc = Delta()..insert('Test\n');
+      final actual = rule.apply(doc, 0, '\nOne\n');
+      expect(actual, isNull);
+    });
+
+    test('preserves line styles', () {
+      final doc = Delta()
+        ..insert('Test')
+        ..insert('\n', ParchmentAttribute.right.toJson());
+      final actual = rule.apply(doc, 4, '\n');
+      final expected = Delta()
+        ..retain(4)
+        ..insert('\n', ParchmentAttribute.right.toJson());
+      expect(actual, expected);
+    });
+  });
+
   group('$PreserveLineStyleOnSplitRule', () {
     final rule = PreserveLineStyleOnSplitRule();
 
@@ -59,8 +80,8 @@ void main() {
     });
   });
 
-  group('$ResetLineFormatOnNewLineRule', () {
-    final rule = const ResetLineFormatOnNewLineRule();
+  group('$ResetHeadingFormatOnNewLineRule', () {
+    final rule = const ResetHeadingFormatOnNewLineRule();
 
     test('applies when line-break is inserted at the end of line', () {
       final doc = Delta()