// src/editor/extensions/firefox-android-backspace.ts
/**
 * Workaround for a CM6 gap on Firefox Android.
 *
 * CM6 has special handling for Chrome Android's IME-routed backspace
 * (the PendingKeys / delayAndroidKey mechanism in @codemirror/view),
 * but no equivalent for Firefox Android. When Firefox Android routes
 * Backspace through the IME pipeline (keydown: Process, keyCode 229),
 * CM6 falls back to watching for DOM mutations. But backspace at the
 * start of a line (<div class="cm-line"><br></div>) produces no DOM
 * mutation, so CM6 sees nothing and drops the event.
 *
 * The fix mimics exactly what CM6 does for Chrome Android:
 * - On beforeinput(deleteContentBackward), call delayAndroidKey('Backspace', 8)
 * - Do NOT preventDefault — let Firefox apply its native backspace
 * - The delayedAndroidKey lock suppresses onSelectionChange and flush
 *   until the next RAF
 * - The RAF calls flush() to read whatever DOM mutations Firefox made
 * - If flush() found no mutations (empty-line case) AND force is true,
 *   CM6 dispatches a synthetic keydown that triggers deleteCharBackward
 *   from defaultKeymap
 *
 * This is the same proven code path Chrome Android uses, just triggered
 * for Firefox Android too. See @codemirror/view input.ts PendingKeys
 * and domobserver.ts delayAndroidKey.
 *
 * Uses internal CM6 API (view.observer.delayAndroidKey). Guarded with
 * runtime checks so a future CM6 refactor degrades gracefully.
 * Tested against @codemirror/view 6.41.1.
 */
import { type Extension } from '@codemirror/state'
import { EditorView } from '@codemirror/view'

/** Detect Firefox Android using the same UA signals CM6 uses internally. */
const isFirefoxAndroid = typeof navigator !== 'undefined'
  && /Firefox/.test(navigator.userAgent)
  && /Android/.test(navigator.userAgent)

export function firefoxAndroidBackspace(): Extension {
  if (!isFirefoxAndroid) return []

  return EditorView.domEventHandlers({
    beforeinput(event, view) {
      if (event.inputType !== 'deleteContentBackward') return false

      const observer = (view as any).observer
      if (typeof observer?.delayAndroidKey !== 'function') return false

      observer.delayAndroidKey('Backspace', 8)

      // Return false: do NOT consume the event. Let Firefox apply its
      // native backspace to the DOM, and let CM6's own beforeinput handler
      // run too (it won't do anything for Firefox since its PendingKeys
      // check is gated on browser.chrome && browser.android).
      return false
    },
  })
}