RenderBadPicture xerror on Xfce startup

Short version: when I start Xfce from xdm, I get a running xfdesktop instance that doesn't work. The wallpaper gets set, then there's something like a flicker (but not really a flicker), and then what should be a usable desktop is there, except it isn't. If I do a "pkill xfdesktop" at that point, the auto-respawned instance of xfdesktop works just fine and it's like nothing was ever wrong. This was first noticed after upgrading everything to xfce-4.16, but since then I've determined that it still occurs after downgrading everything back to 4.14.x. This is what I see in $HOME/.xsession-errors when things don't work properly:

(xfwm4:1519): Gdk-ERROR **: 01:43:48.196: The program 'xfwm4' received an X Window System error.
This probably reflects a bug in the program.
The error was 'RenderBadPicture (invalid Picture parameter)'.
  (Details: serial 8677 error_code 141 request_code 138 (RENDER) minor_code 7)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
xfwm4-Message: 01:43:48.454: Prefer XPresent with Mesa Intel(R) HD Graphics 5500 (BDW GT2)

Other things I've noticed/tried:

1. I have not encountered this issue at all when starting Xfce from runlevel 3 (console login)

2. When starting from xdm, it doesn't happen every time, as if there's some sort of race condition somehow, but I lose the race most of the time (>90%)

3. When I win the race, i.e. when the xfdesktop instance launched initially is working fine, I do not see the RenderBadPicture error in .xsession-errors

I've as yet not tested this on any other machines, so it's entirely possible that there's some obscure corner case involving the graphics hardware on this laptop and/or some other condition specific to this laptop. That testing is planned for the next few days after the holidays quiet down.