fix(placeholder): guard against depth-0 selection in placeholder decoration

packages/super-editor/src/extensions/placeholder/placeholder.js

@@ -29,6 +29,7 @@ export const Placeholder = Extension.create({
       if (plainText !== '') return DecorationSet.empty;
 
       const { $from } = state.selection;
+      if ($from.depth === 0) return DecorationSet.empty;
       const decoration = Decoration.node($from.before(), $from.after(), {
         'data-placeholder': this.options.placeholder,
         class: 'sd-editor-placeholder',

packages/super-editor/src/extensions/placeholder/placeholder.test.js

@@ -0,0 +1,78 @@
+// @ts-check
+import { describe, it, expect } from 'vitest';
+import { Schema } from 'prosemirror-model';
+import { EditorState, Plugin, PluginKey, TextSelection } from 'prosemirror-state';
+import { Decoration, DecorationSet } from 'prosemirror-view';
+
+const schema = new Schema({
+  nodes: {
+    doc: { content: 'paragraph+' },
+    paragraph: { content: 'inline*', group: 'block' },
+    text: { group: 'inline' },
+  },
+});
+
+/**
+ * Recreates the placeholder plugin logic for testing without the Extension system.
+ */
+function createPlaceholderPlugin(placeholderText = 'Type something...') {
+  const applyDecoration = (state) => {
+    const plainText = state.doc.textBetween(0, state.doc.content.size, ' ', ' ');
+    if (plainText !== '') return DecorationSet.empty;
+
+    const { $from } = state.selection;
+    if ($from.depth === 0) return DecorationSet.empty;
+    const decoration = Decoration.node($from.before(), $from.after(), {
+      'data-placeholder': placeholderText,
+      class: 'sd-editor-placeholder',
+    });
+    return DecorationSet.create(state.doc, [decoration]);
+  };
+
+  return new Plugin({
+    key: new PluginKey('placeholder'),
+    state: {
+      init: (_, state) => applyDecoration(state),
+      apply: (tr, oldValue, oldState, newState) => applyDecoration(newState),
+    },
+    props: {
+      decorations(state) {
+        return this.getState(state);
+      },
+    },
+  });
+}
+
+describe('placeholder plugin', () => {
+  it('adds decoration on empty document with cursor in paragraph', () => {
+    const doc = schema.nodes.doc.create(null, [schema.nodes.paragraph.create()]);
+    const plugin = createPlaceholderPlugin();
+    const state = EditorState.create({ doc, schema, plugins: [plugin] });
+
+    const decorations = plugin.getState(state);
+    expect(decorations.find()).toHaveLength(1);
+    expect(decorations.find()[0].type.attrs['data-placeholder']).toBe('Type something...');
+  });
+
+  it('returns empty decorations when document has text', () => {
+    const doc = schema.nodes.doc.create(null, [schema.nodes.paragraph.create(null, [schema.text('Hello')])]);
+    const plugin = createPlaceholderPlugin();
+    const state = EditorState.create({ doc, schema, plugins: [plugin] });
+
+    const decorations = plugin.getState(state);
+    expect(decorations).toBe(DecorationSet.empty);
+  });
+
+  it('does not throw when selection is at doc root (depth 0)', () => {
+    const doc = schema.nodes.doc.create(null, [schema.nodes.paragraph.create()]);
+    const plugin = createPlaceholderPlugin();
+    const state = EditorState.create({ doc, schema, plugins: [plugin] });
+
+    // TextSelection.create at pos 0 lands at doc root (depth 0)
+    const tr = state.tr;
+    tr.setSelection(TextSelection.create(doc, 0));
+    const newState = state.apply(tr);
+
+    expect(plugin.getState(newState)).toBe(DecorationSet.empty);
+  });
+});