Custom fork of ContextThemeWrapper that does some extra stuff to support overriding configurations better.

ContextConfigThemeWrapper.kt

/**
 * Fork of AppCompat ContextThemeWrapper (barely) to correctly support applying override
 * configurations – all the features we don't need are dropped, it always extends the base context's
 * theme, the theme can't be set explicitly, and it adds one critical piece of functionality: the new
 * theme is [rebased][Theme.rebase] after being cloned from the base context's theme.
 */
open class ContextConfigThemeWrapper(
  base: Context,
  private val overrideConfiguration: Configuration
) : ContextWrapper(base) {

  private val layoutInflater: LayoutInflater by lazy(NONE) {
    LayoutInflater.from(baseContext)
      .cloneInContext(this)
  }

  private val resourceContext: Context by lazy(NONE) {
    createConfigurationContext(overrideConfiguration)
  }

  private val _theme: Theme by lazy(NONE) {
    resources.newTheme().apply {
      setTo(baseContext.theme)
      // I don't know why this is required, but without it, child ContextThemeWrappers that use a
      // different theme will fail to resolve attributes specified by this theme.
      if (VERSION.SDK_INT >= VERSION_CODES.Q) {
        rebase()
      }
    }
  }

  // Ensure we're returning assets with the correct configuration.
  override fun getAssets(): AssetManager = resources.assets
  override fun getResources(): Resources = resourceContext.resources

  override fun setTheme(resid: Int) {
    throw UnsupportedOperationException()
  }

  override fun getTheme(): Theme = _theme

  override fun getSystemService(name: String): Any =
    if (LAYOUT_INFLATER_SERVICE == name) layoutInflater else baseContext.getSystemService(name)
}