Description

EquationNode (playground's KaTeX wrapper, a DecoratorNode) had no path that turned a single click into a NodeSelection, so downstream commands (Backspace, copy, Enter, arrow keys) had nothing to act on. A block equation that ends up as the sole root child becomes uninteractive (no caret, no selection, no way to delete), and the inline LaTeX editor's empty-input Backspace was a dead end.

Fix

Mirror the ImageComponent / PollComponent pattern:

• Register CLICK_COMMAND on EquationComponent (COMMAND_PRIORITY_LOW). A click inside the keyed editor-equation DOM sets a NodeSelection; shift+click toggles. The command-level hook fires after lexical's own selection normalization, so the NodeSelection survives the surrounding selectionchange flow.
• Mirror isSelected onto the keyed wrapper as a focused class so the existing .editor-equation.focused outline renders.
• Register KEY_ENTER_COMMAND: NodeSelection on a block equation inserts an empty paragraph after the equation and selects it. Inline equation inserts the paragraph after the wrapper (insertAfter on the inline equation itself would nest paragraph-in-paragraph).
• Drop tabIndex={-1} and role="button" from KatexRenderer. With tabIndex, focus stayed inside the decorator subtree on click and KEY_BACKSPACE_COMMAND short-circuited via $isTargetWithinDecorator(event.target). Removing them matches ImageComponent — focus stays on the contenteditable root, Backspace routes through DELETE_CHARACTER_COMMAND → NodeSelection.deleteNodes().
• Wire EquationEditor's onKeyDown to an onDeleteEmpty callback: when the LaTeX input is already empty and Backspace fires, remove the host node. Block path selects the previous sibling's end (or replaces with an empty paragraph if none); inline path clears the selection and node.remove(true) to preserve the wrapper paragraph.
• Drop the previous "auto-open EquationEditor on lone NodeSelection" effect — it would now fire on every single click. Double-click remains the only entry into edit mode.

Known limitation

Deleting an inline equation (from any path, including main on this branch's base) emits a console error: Lexical node does not exist in active editor state. Avoid using the same node references between nested closures from editorState.read/editor.update.

Rough debugging: the throw lands in $commitPendingUpdates → triggerListeners('update', ...) (LexicalUpdates.ts:714), i.e. after the equation has already been removed and the editor state is committing. One of the registered update listeners reads a node reference that is no longer in the active NodeMap. The trigger reproduces on plain main too, not just on this branch — the fix here doesn't add the bug, it surfaces it more often because the new flows reach this path.

Splitting it off as a follow-up because identifying which listener holds the stale reference means stepping through lexical's internal update / mutation listener chain, and reshaping that chain is well outside the playground change in this PR. Better as a focused follow-up against lexical core once the trigger is isolated.

Closes #8533.

Test plan

• pnpm tsc --noEmit -p tsconfig.json clean.
• Manual on Chrome and Safari:
  • Block equation as sole root child: click → NodeSelection, outline appears. Backspace removes the equation and replaces it with an empty paragraph (caret in the paragraph).
  • Block equation between paragraphs: Enter on the NodeSelection inserts an empty paragraph after the equation, caret inside ready for typing.
  • Inline equation: click → NodeSelection, outline. Backspace removes the equation; caret stays in the surrounding paragraph.
  • Inline equation double-click → input → delete all LaTeX → Backspace removes the host node (wrapper paragraph survives).
  • Double-click remains the only entry into edit mode.