In an experimental branch of compton, I tried using PresentPixmap with xrender to achieve tear free display. I discovered that the XSync fence passed to PresentPixmap never gets triggered, and when I kill compton, the whole server crashes.
I don't have a stack trace, but X server crashed in a function called AttendClient trying to dereference 0x28.
Maybe intel driver only supports fences created via DRI3FenceFromFD. That should've been made clear somewhere (where?), and X server definitely shouldn't crash.
BTW, I suspected the fence because when I don't pass a fence to PresentPixmap, everything seems to work fine.
If you want a testcase, you can try commit 055364431a0706b97c2875d5501dc20dbcf1bdeb of compton.