alpha setting not available/working in xfce panel preferences/appearance with solid color & composting enabled in compton

Submitted by Larry Johnson

Link to original bug (#13701)

Description

This was working previously. but has recently changed.

xorg-server 1.19.3-2
xfce4-panel 4.12.1-1
NVIDIA GLX Module  381.22   [2017-06-30 06:15] [ALPM] upgraded nvidia-ck-sandybridge (1:381.22-3 -> 1:381.22-4)
nVidia GP107 [GeForce GTX 1050 Ti]

Could it be the Compositor?

compton 0.1_beta2.5-8 ( Build Date: Fri 16 Jun 2017 11:12:56 AM PDT)
command: qcompton -b --config /home/ljohnson/.config/compton/compton:0.0.conf -d  :0.0

Attachment 7211, "Screenshot of panel demonstrating problem":

Version: 4.12.1