Tear-free compositing stops working with secondary monitor

Submitted by Marcos Mello

Assigned to Olivier Fourdan @olivier

Link to original bug (#13977)

Description

I run xfwm4-git (g80453816, XPresent and Epoxy enabled, aee8b25a reverted because bug 13954), Intel GM45, Xorg modesetting driver. With only my laptop's primary screen active tear-free compositing is great (thanks for XPresent/OpenGL support!). But when I plug a secondary monitor (VGA or HDMI), I get horrible tearing on both screens.

How can I debug this?