When should a nested server first post a buffer?

[Alan Griffiths]

There's a comment in unity-system-compositor (src/window_manager.cpp)

    // We need to wait for the second frame before marking the session
    // as ready. The first frame posted from sessions is a blank frame.
    // TODO: Solve this issue at its root and remove this workaround

that led me to do some experimenting. And what I discovered is that a
nested server posts a frame when the first surface is *created*.

That is...

1. start a host session: sudo mir_demo_server --vt 2 --arw-file
2. Start a nested session that will be obvious: mir_demo_server
--host-socket /tmp/mir_socket --background-color purple
--custom-compositor adorning --window-manager tiling

    At this point vt2 will have a black display because the nested
    server has yet to post a buffer. (Although there's a titlebar
    visible for its window - which is wrong but not the issue I want to
    raise.)


3. Now create a surface but don't post a buffer: mir_demo_client_basic

    At this point the purple background from the nested server appears.
    (And yes, I checked it is the surface appearing that causes the
    post: "tiling" doesn't paint titlebar surfaces and sticking a
    sleep() after mir_surface_create() makes it clear this is the
    relevant call.)


This seems inconsistent. I could imagine it making sense to post a frame
when starting (but the above comment suggests this isn't desirable).

Since we don't post on startup we really ought to wait until we actually
have content from a client.

Any dissenting opinions?

(And yes, we will need an acceptance for the correct behavior - once
that is agreed.)

PS This also suggests we have unnecessary compositing cycles whenever
surfaces are created.
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