feat: add built-in markdown editor with search jump and typing helpers

lib/markdown_view.dart

@@ -0,0 +1,81 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
+import 'markdown_editing.dart';
+
+/// Highlighted markdown source that is either read-only or editable.
+class MarkdownView extends StatefulWidget {
+  final MarkdownController controller;
+  final bool readOnly;
+  final double fontSize;
+  final FocusNode? focusNode;
+
+  const MarkdownView({
+    super.key,
+    required this.controller,
+    this.readOnly = true,
+    this.fontSize = 16,
+    this.focusNode,
+  });
+
+  @override
+  MarkdownViewState createState() => MarkdownViewState();
+}
+
+class MarkdownViewState extends State<MarkdownView> {
+  final _fieldKey = GlobalKey();
+  final _formatters = [MarkdownInputFormatter()];
+
+  /// Scrolls [offset] into view without focusing the field, so read mode
+  /// doesn't bring up the keyboard.
+  void revealOffset(int offset) {
+    final editable = _findEditable(_fieldKey.currentContext?.findRenderObject());
+    if (editable == null) return;
+    final caret = editable.getLocalRectForCaret(TextPosition(offset: offset));
+    editable.showOnScreen(
+      rect: Rect.fromLTRB(
+          caret.left, caret.top - 80, caret.right, caret.bottom + 80),
+      duration: const Duration(milliseconds: 200),
+      curve: Curves.easeOut,
+    );
+  }
+
+  RenderEditable? _findEditable(RenderObject? node) {
+    if (node is RenderEditable) return node;
+    RenderEditable? found;
+    node?.visitChildren((child) => found ??= _findEditable(child));
+    return found;
+  }
+
+  void _indent({required bool outdent}) {
+    final controller = widget.controller;
+    controller.value = indentLines(controller.value, outdent: outdent);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    // Tab would otherwise move focus out of the editor on desktop.
+    return CallbackShortcuts(
+      bindings: widget.readOnly
+          ? const {}
+          : {
+              const SingleActivator(LogicalKeyboardKey.tab): () =>
+                  _indent(outdent: false),
+              const SingleActivator(LogicalKeyboardKey.tab, shift: true): () =>
+                  _indent(outdent: true),
+            },
+      child: TextField(
+        key: _fieldKey,
+        controller: widget.controller,
+        focusNode: widget.focusNode,
+        readOnly: widget.readOnly,
+        showCursor: !widget.readOnly,
+        expands: true,
+        maxLines: null,
+        decoration: null,
+        inputFormatters: widget.readOnly ? null : _formatters,
+        style: TextStyle(fontSize: widget.fontSize),
+      ),
+    );
+  }
+}