Setup:
- Skylake i5 (BXT had same issue too)
- Git versions of:
  - kernel (support atomic modesetting)
  - X server / Xwayland
  - Mesa
  - Weston
  (latest releases may be enough, but hasn't been tested)
- New enough dependencies for above, so that modifiers are supported
- GL application supporting fullscreen in both X and Wayland:
  # apt install glmark2 glmar2-wayland

Test-case:
- Start Weston with --xwayland
- Start fullscreen Wayland GL application:
  glmark2-wayland --fullscreen
- Start fullscreen X GL application:
  glmark2 --fullscreen
- Start windowed X GL application:
  glmark2

Expected outcome:
- Fullscreen X GL application works as well through XWayland, as native Wayland GL applications

Actual outcome:
- Wayland GL application fullscreen window works fine
- X GL application fullscreen window never becomes visible
  - Or with some applications, it appears on random runs, after random delay, but with Weston panel on top of it instead of as fullscreen
- Windowed X GL application windows work fine

Notes:
- Same thing regardless of whether application uses GLES or GL, glX or EGL
- With some fullscreen GL benchmarks, the window appears after a while, but in that case it's below the Weston panel i.e. not really fullscreen
  - Whether and when the (non-fullscreen) window appears seems *completely random*
  - I've seen this most reliably with Ungine Heaven and Valley benchmarks

I haven't tried XWayland before, but I assume it must be a regression.

Xprop and Xwininfo report following for the running invisible glmark2 window:
---------------------
_NET_WM_DESKTOP(CARDINAL) = 0
WM_STATE(WM_STATE):
		window state: Normal
		icon window: 0x0
WM_PROTOCOLS(ATOM): protocols  WM_DELETE_WINDOW
WM_NAME(STRING) = "glmark2 2014.03+git20150611.fa71af2d"
_NET_WM_STATE(ATOM) = _NET_WM_STATE_FULLSCREEN
---------------------
  Absolute upper-left X:  0
  Absolute upper-left Y:  0
  Relative upper-left X:  0
  Relative upper-left Y:  0
  Width: 1920
  Height: 1080
  Depth: 24
  Visual: 0x24
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x400001 (not installed)
  Bit Gravity State: ForgetGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +0+0  -0+0  -0-0  +0-0
  -geometry 1920x1080+0+0
---------------------
(This was on FullHD monitor.)