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

lib/src/editor/block_component/heading_block_component/heading_command_shortcut.dart

@@ -74,8 +74,11 @@ KeyEventResult _toggleAttribute(
       node.type == HeadingBlockKeys.type &&
           node.attributes[HeadingBlockKeys.level] == level;
 
-  final delta = (node.delta ?? Delta()).toJson();
-
+  // The delta is read per node inside the callback. Reading it once from
+  // `selection.start` and reusing it here wrote the first block's text over
+  // every block in a multi-block selection, destroying the rest -- and this
+  // shortcut path has no single-selection gate at all, so it was reachable
+  // with any selection. See heading_toolbar_items.dart (2026-07-27).
   editorState.formatNode(
     selection,
     (node) => node.copyWith(
@@ -86,7 +89,7 @@ KeyEventResult _toggleAttribute(
             node.attributes[blockComponentBackgroundColor],
         blockComponentTextDirection:
             node.attributes[blockComponentTextDirection],
-        blockComponentDelta: delta,
+        blockComponentDelta: (node.delta ?? Delta()).toJson(),
       },
     ),
   );

lib/src/editor/toolbar/desktop/items/heading_toolbar_items.dart

@@ -23,28 +23,40 @@ class _HeadingToolbarItem extends ToolbarItem {
             final node = editorState.getNodeAtPath(selection.start.path)!;
             final isHighlight =
                 node.type == 'heading' && node.attributes['level'] == level;
-            final delta = (node.delta ?? Delta()).toJson();
             final child = SVGIconItemWidget(
               iconName: 'toolbar/h$level',
               isHighlight: isHighlight,
               highlightColor: highlightColor,
               iconColor: iconColor,
-              onPressed: () => editorState.formatNode(
-                selection,
-                (node) => node.copyWith(
-                  type: isHighlight
-                      ? ParagraphBlockKeys.type
-                      : HeadingBlockKeys.type,
-                  attributes: {
-                    HeadingBlockKeys.level: level,
-                    blockComponentBackgroundColor:
-                        node.attributes[blockComponentBackgroundColor],
-                    blockComponentTextDirection:
-                        node.attributes[blockComponentTextDirection],
-                    blockComponentDelta: delta,
-                  },
-                ),
-              ),
+              onPressed: () {
+                // Both the selection and each node's delta are read HERE, not
+                // captured when this item was built. Hoisting them out was real
+                // data loss: `formatNode` runs the callback for every node in
+                // the selection, so one block's text was written over all of
+                // them and the others' writing was destroyed (2026-07-27). The
+                // build-time capture also went stale whenever the selection
+                // grew while the toolbar stayed on screen.
+                final selection = editorState.selection;
+                if (selection == null) {
+                  return;
+                }
+                editorState.formatNode(
+                  selection,
+                  (node) => node.copyWith(
+                    type: isHighlight
+                        ? ParagraphBlockKeys.type
+                        : HeadingBlockKeys.type,
+                    attributes: {
+                      HeadingBlockKeys.level: level,
+                      blockComponentBackgroundColor:
+                          node.attributes[blockComponentBackgroundColor],
+                      blockComponentTextDirection:
+                          node.attributes[blockComponentTextDirection],
+                      blockComponentDelta: (node.delta ?? Delta()).toJson(),
+                    },
+                  ),
+                );
+              },
             );
 
             if (tooltipBuilder != null) {

lib/src/editor/toolbar/desktop/items/paragraph_toolbar_item.dart

@@ -10,25 +10,33 @@ final ToolbarItem paragraphItem = ToolbarItem(
     final selection = editorState.selection!;
     final node = editorState.getNodeAtPath(selection.start.path)!;
     final isHighlight = node.type == 'paragraph';
-    final delta = (node.delta ?? Delta()).toJson();
     final child = SVGIconItemWidget(
       iconName: 'toolbar/text',
       isHighlight: isHighlight,
       highlightColor: highlightColor,
       iconColor: iconColor,
-      onPressed: () => editorState.formatNode(
-        selection,
-        (node) => node.copyWith(
-          type: ParagraphBlockKeys.type,
-          attributes: {
-            blockComponentDelta: delta,
-            blockComponentBackgroundColor:
-                node.attributes[blockComponentBackgroundColor],
-            blockComponentTextDirection:
-                node.attributes[blockComponentTextDirection],
-          },
-        ),
-      ),
+      // Read the selection and each node's delta at press time. See the note in
+      // heading_toolbar_items.dart -- hoisting the delta out of this callback
+      // wrote one block's text over every block in the selection.
+      onPressed: () {
+        final selection = editorState.selection;
+        if (selection == null) {
+          return;
+        }
+        editorState.formatNode(
+          selection,
+          (node) => node.copyWith(
+            type: ParagraphBlockKeys.type,
+            attributes: {
+              blockComponentDelta: (node.delta ?? Delta()).toJson(),
+              blockComponentBackgroundColor:
+                  node.attributes[blockComponentBackgroundColor],
+              blockComponentTextDirection:
+                  node.attributes[blockComponentTextDirection],
+            },
+          ),
+        );
+      },
     );
 
     if (tooltipBuilder != null) {

test/new/block_component/heading_block_component/heading_command_shortcut_test.dart

@@ -149,5 +149,45 @@ void main() async {
       node = editorState.getNodeAtPath([0])!;
       expect(node.type, ParagraphBlockKeys.type);
     });
+
+    // Regression test for real data loss on a real page, 2026-07-27.
+    //
+    // The delta was read ONCE from the block at `selection.start` and then
+    // written into every block the format callback ran for. Toggling a heading
+    // over a multi-block selection therefore stamped the first block's text
+    // over all the others and destroyed their writing. This shortcut path has
+    // no single-selection gate, so any selection could trigger it.
+    test('toggling a heading over several blocks keeps each blocks own text',
+        () {
+      final document = Document.blank()
+        ..addParagraph(initialText: 'first')
+        ..addParagraph(initialText: 'second')
+        ..addParagraph(initialText: 'third');
+      final editorState = EditorState(document: document);
+
+      editorState.selection = Selection(
+        start: Position(path: [0]),
+        end: Position(path: [2], offset: 'third'.length),
+      );
+      toggleH1.execute(editorState);
+
+      final texts = editorState.document.root.children
+          .map((n) => n.delta?.toPlainText())
+          .toList();
+      expect(texts, ['first', 'second', 'third']);
+      expect(
+        editorState.document.root.children.map((n) => n.type),
+        everyElement(HeadingBlockKeys.type),
+      );
+
+      // ...and back again, which is the same callback with the other type.
+      toggleH1.execute(editorState);
+      expect(
+        editorState.document.root.children
+            .map((n) => n.delta?.toPlainText())
+            .toList(),
+        ['first', 'second', 'third'],
+      );
+    });
   });
 }