val popup = PopupWindow(...)

val container = popup.contentView.takeIf { it.layoutParams is WindowManager.LayoutParams }
    ?: (popup.contentView.parent as? View)?.takeIf { it.layoutParams is WindowManager.LayoutParams }
    ?: (popup.contentView.parent.parent as? View)?.takeIf { it.layoutParams is WindowManager.LayoutParams }
    ?: throw IllegalStateException("NO WindowManager.LayoutParams!")

val wm = context.getSystemService(Context.WINDOW_SERVICE) as WindowManager
val lp = container.layoutParams as WindowManager.LayoutParams
lp.flags = lp.flags or WindowManager.LayoutParams.FLAG_DIM_BEHIND
lp.dimAmount = 0.3f
wm.updateViewLayout(container, lp)