Xwayland clients not displayed unless something else is running

[AlanGriffiths]

Extracted from canonical/mir-kiosk#5 (more discussion and scenarios there).

Run Mir from ssh into test system:

ssh(1)
$ sudo --preserve-env miral-shell --vt 4 --arw-file
ssh(2)
$ sudo --preserve-env WAYLAND_DEBUG=1 Xwayland -wr :1
ssh(3)
$ sudo DISPLAY=:1 xeyes
Expect: xeyes appears
Actual: nothing appears

---

Running an additional program seems to fix this. E.g.
$ mir_demo_client_multiwin& sudo --preserve-env WAYLAND_DEBUG=1 Xwayland -wr :1

This suggests there's a problem getting the compositor to use the first buffer which should result in a frame notification back to the Xwayland client.

[AlanGriffiths]

@gerboland I think I understand the problem: The surface is being added to the scene but the scene's observer doesn't see a buffer posted.

The reason:

1. the buffer is posted;
2. the surface created; and,
3. only then, the observer is added.

(That sequence is something we enabled recently.)

The reason having another app around helps is that the z-order change triggers compositing. I just have to figure out the right twisty little function to fix.