fix: changing block type over a multi-block selection destroys text

frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/heading/heading_toolbar_item.dart

@@ -94,7 +94,11 @@ final headingsToolbarItem = ToolbarItem(
                     node.attributes[blockComponentBackgroundColor],
                 blockComponentTextDirection:
                     node.attributes[blockComponentTextDirection],
-                blockComponentDelta: delta,
+                // Per node: this callback runs for every node in the
+                // selection, and the `delta` captured above belongs only to
+                // the block at `selection.start`. The single-node path in the
+                // branch above may keep using it.
+                blockComponentDelta: (node.delta ?? Delta()).toJson(),
               },
             ),
           );

frontend/appflowy_flutter/lib/plugins/document/presentation/editor_plugins/toolbar_item/text_heading_toolbar_item.dart

@@ -229,14 +229,16 @@ enum TextHeadingCommand {
 
 void formatNodeToText(EditorState editorState) {
   final selection = editorState.selection!;
-  final node = editorState.getNodeAtPath(selection.start.path)!;
-  final delta = (node.delta ?? Delta()).toJson();
+  // Read the delta from the node being transformed. `formatNode` runs this
+  // callback for every node in the selection, so reading it once from
+  // `selection.start` wrote the first block's text over every other block and
+  // destroyed their content.
   editorState.formatNode(
     selection,
     (node) => node.copyWith(
       type: ParagraphBlockKeys.type,
       attributes: {
-        blockComponentDelta: delta,
+        blockComponentDelta: (node.delta ?? Delta()).toJson(),
         blockComponentBackgroundColor:
             node.attributes[blockComponentBackgroundColor],
         blockComponentTextDirection:

frontend/appflowy_flutter/test/unit_test/document/editor_plugins/toolbar_item/multi_block_turn_into_test.dart

@@ -0,0 +1,134 @@
+import 'package:appflowy/plugins/document/presentation/editor_plugins/actions/block_action_option_cubit.dart';
+import 'package:appflowy/plugins/document/presentation/editor_plugins/toolbar_item/text_heading_toolbar_item.dart';
+import 'package:appflowy_editor/appflowy_editor.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+// Changing the block type of a multi-block selection used to leave every block
+// holding the FIRST block's text, destroying the others' content.
+//
+// `formatNodeToText` read the delta once from `selection.start` and reused it
+// inside the `formatNode` callback -- which runs for every node in the
+// selection. The second test below fails on the unfixed code with
+// ['first paragraph', 'first paragraph'].
+//
+// The `turnIntoBlock` cases are guards: that path reads the delta per node
+// already, and these keep it that way.
+
+Document _twoParagraphs() => Document(
+      root: pageNode(
+        children: [
+          paragraphNode(text: 'first paragraph'),
+          paragraphNode(text: 'second paragraph'),
+        ],
+      ),
+    );
+
+/// Selects from the start of the first block to the end of the last.
+Selection _selectAll(EditorState editorState) {
+  final children = editorState.document.root.children;
+  return Selection(
+    start: Position(path: children.first.path),
+    end: Position(
+      path: children.last.path,
+      offset: children.last.delta?.length ?? 0,
+    ),
+  );
+}
+
+List<String> _texts(EditorState editorState) =>
+    editorState.document.root.children
+        .map((n) => n.delta?.toPlainText() ?? '')
+        .toList();
+
+void main() {
+  group('block type change across a multi-block selection', () {
+    test('turning two paragraphs into headings keeps each ones own text',
+        () async {
+      final editorState = EditorState(document: _twoParagraphs());
+      editorState.selection = _selectAll(editorState);
+
+      await BlockActionOptionCubit.turnIntoBlock(
+        HeadingBlockKeys.type,
+        editorState.getNodeAtPath([0])!,
+        editorState,
+        level: 3,
+        keepSelection: true,
+      );
+
+      expect(_texts(editorState), ['first paragraph', 'second paragraph']);
+      expect(
+        editorState.document.root.children.map((n) => n.type),
+        everyElement(HeadingBlockKeys.type),
+      );
+    });
+
+    test('turning two headings back into text keeps each ones own text',
+        () async {
+      final editorState = EditorState(
+        document: Document(
+          root: pageNode(
+            children: [
+              headingNode(level: 3, text: 'first paragraph'),
+              headingNode(level: 3, text: 'second paragraph'),
+            ],
+          ),
+        ),
+      );
+      editorState.selection = _selectAll(editorState);
+
+      formatNodeToText(editorState);
+      await Future<void>.delayed(Duration.zero);
+
+      expect(_texts(editorState), ['first paragraph', 'second paragraph']);
+    });
+
+    test('an alignment attribute does not reintroduce the duplication',
+        () async {
+      final editorState = EditorState(
+        document: Document(
+          root: pageNode(
+            children: [
+              paragraphNode(
+                text: 'first paragraph',
+                attributes: {blockComponentAlign: 'justify'},
+              ),
+              paragraphNode(
+                text: 'second paragraph',
+                attributes: {blockComponentAlign: 'justify'},
+              ),
+            ],
+          ),
+        ),
+      );
+      editorState.selection = _selectAll(editorState);
+
+      await BlockActionOptionCubit.turnIntoBlock(
+        HeadingBlockKeys.type,
+        editorState.getNodeAtPath([0])!,
+        editorState,
+        level: 3,
+        keepSelection: true,
+      );
+      expect(_texts(editorState), ['first paragraph', 'second paragraph']);
+
+      editorState.selection = _selectAll(editorState);
+      formatNodeToText(editorState);
+      await Future<void>.delayed(Duration.zero);
+      expect(_texts(editorState), ['first paragraph', 'second paragraph']);
+    });
+
+    test('a single-block selection is unaffected', () async {
+      final editorState = EditorState(document: _twoParagraphs());
+      editorState.selection = Selection.single(path: [1], startOffset: 0);
+
+      await BlockActionOptionCubit.turnIntoBlock(
+        HeadingBlockKeys.type,
+        editorState.getNodeAtPath([1])!,
+        editorState,
+        level: 1,
+      );
+
+      expect(_texts(editorState), ['first paragraph', 'second paragraph']);
+    });
+  });
+}