Bug: Backspace at the start of the first block throws "window object not found" in a headless editor #9849

Lexical version: 0.53.0 (the code path is unchanged on main)

Backspace (DELETE_CHARACTER_COMMAND with isBackward = true) at the start of the first block throws Error: window object not found in a headless editor, so the block is never collapsed (e.g. a quote is not turned into a paragraph). The same keystroke works when there is a previous block, because the cross-block merge is resolved in the model.

We hit this running @lexical/headless inside JavaScriptCore (no DOM) as the model behind a native iOS text view; it also reproduces in Node/Bun.

Steps To Reproduce

1. Create a headless editor with registerRichText and a QuoteNode.
2. Put the caret at the start of a quote that is the first child of the root.
3. Dispatch DELETE_CHARACTER_COMMAND with true.

Link to code example:

import { createHeadlessEditor } from "@lexical/headless";
import { $createQuoteNode, QuoteNode, registerRichText } from "@lexical/rich-text";
import {
  $createParagraphNode,
  $createTextNode,
  $getRoot,
  DELETE_CHARACTER_COMMAND,
} from "lexical";

function run(withPreviousParagraph) {
  const editor = createHeadlessEditor({
    nodes: [QuoteNode],
    onError: (error) => {
      throw error;
    },
  });
  registerRichText(editor);
  editor.update(
    () => {
      const quote = $createQuoteNode().append($createTextNode("quote"));
      if (withPreviousParagraph)
        $getRoot().append($createParagraphNode().append($createTextNode("before")));
      $getRoot().append(quote);
      quote.selectStart();
    },
    { discrete: true },
  );
  try {
    editor.update(() => editor.dispatchCommand(DELETE_CHARACTER_COMMAND, true), {
      discrete: true,
    });
    console.log(withPreviousParagraph, editor.read(() => $getRoot().getChildren().map((n) => n.getType())));
  } catch (error) {
    console.log(withPreviousParagraph, String(error));
  }
}

run(false); // false 'Error: window object not found'
run(true);  // true [ 'paragraph' ]

The current behavior

RangeSelection.deleteCharacter finds no adjacent block to merge with, then calls $extendSelectionForDeletion(this, isBackward, 'character'). That function calls getDOMSelection(getWindow(editor)), and getWindow throws window object not found because editor._window is null in a headless editor. The else if (isBackward && anchor.offset === 0) branch that runs $collapseAtStart is never reached.

The expected behavior

The quote collapses to a paragraph, as it does in the browser. $extendSelectionForDeletion already returns false when there is no DOM selection (if (!domSelection || ...) return false;), so returning false when the editor has no window (instead of letting getWindow throw) would let deleteCharacter continue to the model-only $collapseAtStart path.

Impact of fix

Affects any headless use of the rich text commands (server-side processing, tests, native shells that use Lexical as the document model). Backspace at the very start of a document is a common keystroke, and today it throws instead of being a no-op or collapsing the block.