Workspace Switcher uses wrong aspect ratio with Compiz (issue #119 still kicking)

Issue 119 which was closed with commit 8f310b86 seems to still exist to some degree.

The following three screenshots from Archlinux, using xfce-panel 4.18.4, illustrates the issue:

Xfce panel with 4 workspaces, using xfwm4

Screenshot 1

Switching to Compiz 0.9.14.2 still gives 4 workspaces, but they are compressed into the space of one of the previous four workspaces (Compiz version 0.8.18 has the same issue)

Screenshot 2

Switching back to xfwm4 again, there is now only one workspace, not distorted, same size as in screenshot 1

Screenshot 3

I'm not sure if the fault is with xfce-panel or Compiz, but something is amiss with the workspace management.