Thanks to the new ~arch xorg-x11 6.8.0, the composite extension is now being used by quite a few people. As an experiment, I went and compiled the latest version of metacity from source. It seems to work perfectly with the compositor enabled (on amd64 and x86, so far). This would also resolve bug #61739.
Created attachment 39295 [details] x11-wm/metacity/metacity-2.8.4.ebuild This ebuild uses a local compositor USE to enable the compositor. This means the default behavior would be identical to the 2.8.1 ebuild already in portage.
the compositor is known buggy & this is still a devel release. In the 2.8 release i will disable the compositor by default for the first reason.