Refactor focusing to contextLine

playground/frontend/lib/pages/embedded_playground/widgets/embedded_editor.dart

@@ -37,7 +37,6 @@ class EmbeddedEditor extends StatelessWidget {
     return SnippetEditor(
       controller: snippetController,
       isEditable: isEditable,
-      goToContextLine: false,
     );
   }
 }

playground/frontend/lib/pages/standalone_playground/widgets/editor_textarea_wrapper.dart

@@ -56,7 +56,6 @@ class CodeTextAreaWrapper extends StatelessWidget {
                   child: SnippetEditor(
                     controller: snippetController,
                     isEditable: true,
-                    goToContextLine: true,
                   ),
                 ),
                 Positioned(

playground/frontend/playground_components/lib/src/controllers/snippet_editing_controller.dart

@@ -16,6 +16,8 @@
  * limitations under the License.
  */
 
+import 'dart:math';
+
 import 'package:flutter/widgets.dart';
 import 'package:flutter_code_editor/flutter_code_editor.dart';
 import 'package:get_it/get_it.dart';
@@ -77,6 +79,7 @@ class SnippetEditingController extends ChangeNotifier {
     codeController.removeListener(_onCodeControllerChanged);
     setSource(example.source);
     _applyViewOptions(viewOptions);
+    _toStartOfContextLineIfAny();
     codeController.addListener(_onCodeControllerChanged);
 
     notifyListeners();
@@ -100,6 +103,31 @@ class SnippetEditingController extends ChangeNotifier {
     }
   }
 
+  void _toStartOfContextLineIfAny() {
+    final contextLine1Based = selectedExample?.contextLine;
+
+    if (contextLine1Based == null) {
+      return;
+    }
+
+    _toStartOfFullLine(max(contextLine1Based - 1, 0));
+  }
+
+  void _toStartOfFullLine(int line) {
+    if (line >= codeController.code.lines.length) {
+      return;
+    }
+
+    final fullPosition = codeController.code.lines.lines[line].textRange.start;
+    final visiblePosition = codeController.code.hiddenRanges.cutPosition(
+      fullPosition,
+    );
+
+    codeController.selection = TextSelection.collapsed(
+      offset: visiblePosition,
+    );
+  }
+
   Example? get selectedExample => _selectedExample;
 
   ExampleLoadingDescriptor? get descriptor => _descriptor;

playground/frontend/playground_components/lib/src/models/example_base.dart

@@ -50,6 +50,8 @@ extension ExampleTypeToString on ExampleType {
 /// These objects are fetched as lists from [ExampleRepository].
 class ExampleBase with Comparable<ExampleBase>, EquatableMixin {
   final Complexity? complexity;
+
+  /// Index of the line to focus, 1-based.
   final int contextLine;
   final String description;
   final bool isMultiFile;

playground/frontend/playground_components/lib/src/widgets/snippet_editor.dart

@@ -16,31 +16,120 @@
  * limitations under the License.
  */
 
-import 'package:flutter/widgets.dart';
+import 'dart:math';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/scheduler.dart';
+import 'package:flutter_code_editor/flutter_code_editor.dart';
 
 import '../controllers/snippet_editing_controller.dart';
-import 'editor_textarea.dart';
+import '../theme/theme.dart';
 
-class SnippetEditor extends StatelessWidget {
+class SnippetEditor extends StatefulWidget {
   final SnippetEditingController controller;
   final bool isEditable;
-  final bool goToContextLine;
 
-  const SnippetEditor({
+  SnippetEditor({
     required this.controller,
     required this.isEditable,
-    required this.goToContextLine,
-  });
+  }) : super(
+    // When the example is changed, will scroll to the context line again.
+    key: ValueKey(controller.selectedExample),
+  );
+
+  @override
+  State<SnippetEditor> createState() => _SnippetEditorState();
+}
+
+class _SnippetEditorState extends State<SnippetEditor> {
+  bool _didAutoFocus = false;
+  final _focusNode = FocusNode();
+  final _scrollController = ScrollController();
+
+  @override
+  void didChangeDependencies() {
+    super.didChangeDependencies();
+
+    if (!_didAutoFocus) {
+      _didAutoFocus = true;
+      SchedulerBinding.instance.addPostFrameCallback((_) {
+        if (mounted) {
+          _scrollSoCursorIsOnTop();
+        }
+      });
+    }
+  }
+
+  void _scrollSoCursorIsOnTop() {
+    _focusNode.requestFocus();
+
+    final position = max(widget.controller.codeController.selection.start, 0);
+    final characterOffset = _getLastCharacterOffset(
+      text: widget.controller.codeController.text.substring(0, position),
+      style: kLightTheme.extension<BeamThemeExtension>()!.codeRootStyle,
+    );
+
+    _scrollController.jumpTo(
+      min(
+        characterOffset.dy,
+        _scrollController.position.maxScrollExtent,
+      ),
+    );
+  }
+
+  @override
+  void dispose() {
+    _focusNode.dispose();
+    super.dispose();
+  }
 
   @override
   Widget build(BuildContext context) {
-    return EditorTextArea(
-      codeController: controller.codeController,
-      sdk: controller.sdk,
-      enabled: !(controller.selectedExample?.isMultiFile ?? false),
-      example: controller.selectedExample,
-      isEditable: isEditable,
-      goToContextLine: goToContextLine,
+    final ext = Theme.of(context).extension<BeamThemeExtension>()!;
+    final isMultiFile = widget.controller.selectedExample?.isMultiFile ?? false;
+    final isEnabled = widget.isEditable && !isMultiFile;
+
+    return Semantics(
+      container: true,
+      enabled: isEnabled,
+      label: 'widgets.codeEditor.label',
+      multiline: true,
+      readOnly: isEnabled,
+      textField: true,
+      child: FocusScope(
+        node: FocusScopeNode(canRequestFocus: isEnabled),
+        child: CodeTheme(
+          data: ext.codeTheme,
+          child: Container(
+            color: ext.codeTheme.styles['root']?.backgroundColor,
+            child: SingleChildScrollView(
+              controller: _scrollController,
+              child: CodeField(
+                key: ValueKey(widget.controller.codeController),
+                controller: widget.controller.codeController,
+                enabled: isEnabled,
+                focusNode: _focusNode,
+                textStyle: ext.codeRootStyle,
+              ),
+            ),
+          ),
+        ),
+      ),
     );
   }
 }
+
+Offset _getLastCharacterOffset({
+  required String text,
+  required TextStyle style,
+}) {
+  final textPainter = TextPainter(
+    textDirection: TextDirection.ltr,
+    text: TextSpan(text: text, style: style),
+  )..layout();
+
+  return textPainter.getOffsetForCaret(
+    TextPosition(offset: text.length),
+    Rect.zero,
+  );
+}