preserve user entered newlines in message parsing

Author: jhass

lib/src/markdown_extensions.dart

@@ -73,3 +73,13 @@ class DiasporaAutolinkSyntax extends InlineSyntax {
     return true;
   }
 }
+
+class SingleNewlineBreakSyntax extends InlineSyntax {
+  SingleNewlineBreakSyntax() : super(r'(?<!\\)(?<! {2})\n', startCharacter: 0x0A);
+
+  @override
+  bool onMatch(InlineParser parser, Match match) {
+    parser.addNode(Element.empty('br'));
+    return true;
+  }
+}

lib/src/messages.dart

@@ -73,6 +73,7 @@ class Message extends StatelessWidget with LocalizationHelpers {
               final people = mentionedPeople;
               return people != null ? people[diasporaId] : null;
             }),
+            mde.SingleNewlineBreakSyntax(),
             mde.DiasporaAutolinkSyntax()
           ],
         ),

test/messages_markdown_breaks_test.dart

@@ -0,0 +1,56 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:insporation/src/client.dart';
+import 'package:insporation/src/markdown_extensions.dart' as mde;
+import 'package:markdown/markdown.dart' as md;
+
+String renderMessageMarkdown(String body, {Map<String, Person>? mentionedPeople}) {
+  return md.markdownToHtml(
+    body,
+    blockSyntaxes: [
+      md.TableSyntax(),
+      md.FencedCodeBlockSyntax(),
+    ],
+    inlineSyntaxes: [
+      md.InlineHtmlSyntax(),
+      mde.SuperscriptSyntax(),
+      mde.SubscriptSyntax(),
+      md.StrikethroughSyntax(),
+      md.AutolinkExtensionSyntax(),
+      mde.TagLinkSyntax(),
+      mde.MentionLinkSyntax((diasporaId, inlineName) {
+        final people = mentionedPeople;
+        return people != null ? people[diasporaId] : null;
+      }),
+      mde.SingleNewlineBreakSyntax(),
+      mde.DiasporaAutolinkSyntax(),
+    ],
+  );
+}
+
+void main() {
+  test('single newline in paragraph renders as html break', () {
+    final html = renderMessageMarkdown('hello\nworld');
+
+    expect(html, contains('<br'));
+  });
+
+  test('fenced code block newlines are not converted to html breaks', () {
+    final html = renderMessageMarkdown('```\nhello\nworld\n```');
+
+    expect(html, contains('<pre><code>hello\nworld\n</code></pre>'));
+    expect(html, isNot(contains('<code>hello<br')));
+  });
+
+  test('hard breaks via trailing spaces keep existing behavior', () {
+    final html = renderMessageMarkdown('hello  \nworld');
+
+    expect(html, contains('<p>hello<br />\nworld</p>'));
+  });
+
+  test('inline code does not get newline break conversion', () {
+    final html = renderMessageMarkdown('`hello\nworld`');
+
+    expect(html, contains('<p><code>hello world</code></p>'));
+    expect(html, isNot(contains('<code>hello<br')));
+  });
+}