[xubuntu] Switched to HWE stack, now brightness keys don't do anything

I'm running Xubuntu 16.04 on a MacBook Pro 9,1. I switched to the HWE stack by running this command -
Now my display brightness keys no longer change the display brightness, despite the setting being checked in Xfce Power Manager, and the brightness notification showing the change.

My command-line script to control the brightness still works, but it's not very convenient.

How to get Xfce Power Manager able to control the brightness again? That is, without switching back to the non-HWE stack.

Thanks for any suggestions.