fix(mobile): bound preview scanning and honor escaped backticks

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/lib/features/channels/message_content/preview_text.dart

@@ -145,8 +145,10 @@ String messagePreviewSource(String content, {required int maxLines}) {
 }
 
 /// The first `maxLines + 1` lines that render, trimmed and without block
-/// syntax, read lazily so the rest of [content] is never scanned. [code] marks
+/// syntax, read lazily so the rest of [content] is never scanned. Lines that
-/// a line inside a fenced block, and [end] is its end offset in [content].
+/// don't render still cost a scan, so reading also stops after
+/// [_maxScanLength] characters. [code] marks a line inside a fenced block, and
+/// [end] is its end offset in [content].
 Iterable<({String text, bool code, int end})> _keptLines(
   String content,
   int maxLines,
@@ -154,7 +156,9 @@ Iterable<({String text, bool code, int end})> _keptLines(
   var kept = 0;
   var fenceOpen = false;
   var start = 0;
-  while (start <= content.length && kept <= maxLines) {
+  while (start <= content.length &&
+      start <= _maxScanLength &&
+      kept <= maxLines) {
     var end = content.indexOf('\n', start);
     if (end < 0) end = content.length;
     // A long line wraps past the visible lines well before this length.
@@ -183,6 +187,8 @@ Iterable<({String text, bool code, int end})> _keptLines(
 }
 
 const _maxLineLength = 2000;
+const _maxScanLength = 8 * _maxLineLength;
+const _backslash = 0x5C;
 const _backtick = 0x60;
 
 final _rule = RegExp(r'^([-*_])(?:\s*\1){2,}$');
@@ -301,13 +307,18 @@ class _PreviewBuilder {
       piece is PreviewText ? text(piece.text) : pieces.add(piece);
 
   /// One line of prose: inline code is literal, everything else is inline
-  /// Markdown. A code span opens at a run of backticks and closes at the next
+  /// Markdown. A code span opens at an unescaped run of backticks and closes
-  /// run of the same length; a run with no match is literal.
+  /// at the next run of the same length; a run with no match is literal.
   void line(String line) {
     var prose = 0;
     var cursor = 0;
     while (cursor < line.length) {
-      if (line.codeUnitAt(cursor) != _backtick) {
+      final unit = line.codeUnitAt(cursor);
+      if (unit == _backslash) {
+        cursor += 2;
+        continue;
+      }
+      if (unit != _backtick) {
         cursor++;
         continue;
       }

mobile/test/features/channels/message_content/preview_text_test.dart

@@ -62,6 +62,7 @@ void main() {
       ('escaped emphasis', r'\*literal\* and \_x\_', '*literal* and _x_'),
       ('escaped backslash', r'a \\ b', r'a \ b'),
       ('escaped link opener', r'\[docs](https://x)', '[docs](https://x)'),
+      ('escaped backticks', r'\`literal\` and `code`', '`literal` and code'),
     ]) {
       test(name, () => expect(_text(source), expected));
     }
@@ -99,6 +100,15 @@ void main() {
       expect(_text('# a\n\n---\n- b\nc\nd'), 'a\nb\nc');
     });
 
+    test('stops reading after a long run of lines that do not render', () {
+      final content = '\n' * 1000000 + 'late';
+      expect(_text(content), '');
+      expect(
+        messagePreviewSource(content, maxLines: 2).length,
+        lessThan(20000),
+      );
+    });
+
     test('caps a very long line', () {
       expect(_text('x' * 5000).length, 2000);
     });