• Pekka Paalanen's avatar
    protocol: make get_subsurface double-buffered · de24f4dd
    Pekka Paalanen authored
    The existing specification was not explicitly clear on when
    wl_subcompositor.get_subsurface request actually adds the sub-surface to
    the parent in the compositor's scenegraph. The implicit assumption was
    that this happens immediately, but it was not written anywhere.
    
    If it happens immediately, the client doing things in a wrong order may
    cause a glitch on screen. Particularly, if the wl_surface B that is
    going to be a sub-surface for wl_surface A (the parent) already has a
    buffer committed, and the parent surface is mapped, then get_subsurface
    will (may?) cause wl_surface B to become mapped immediately. That leaves
    no time to set up the sub-surface z-order or position before mapping,
    hence there can be a visible glitch.
    
    The way to avoid that, given that the parent surface is mapped, is to
    not commit a buffer to wl_surface B until all the sub-surface setup is
    done.
    
    However, doing the sub-surface setup always requires a wl_surface.commit
    on the parent surface unless the defaults happen to be correct.
    
    To make setting up a subsurface slightly easier by removing one
    possibility for a glitch, this patch amends the specification to require
    a wl_surface.commit on the parent surface for get_subsurface to
    complete. The sub-surface cannot become mapped before a parent commit.
    
    This change may break existing clients that relied on the glitchy
    sequence to not need a parent surface commit to map the sub-surface.
    However, presumably all uses would at least issue a
    wl_subsurface.set_position, which requires a parent surface commit to
    apply. That would guarantee that there is a parent surface commit after
    get_subsurface, and so reduces the chances of breaking anything.
    
    In other cases, this change may simply remove a possibility for the
    glitch.
    
    This patch also adds a note about changing wl_surface.commit behaviour
    on wl_subcompositor.get_subsurface. (That could be a separate patch.)
    
    The behaviour of wl_subsurface.destroy remains as specified, even though
    it is now slightly asymmetrical to get_subsurface. This is emphasized by
    adding the word "immediately". The effects of destruction were already
    explicitly documented, as is the way to achieve synchronized unmapping,
    so changing destruction behaviour would likely be more disruptive, and
    also open up more corner cases (what would happen between destroy and
    unmapping?).
    
    Bug: https://phabricator.freedesktop.org/T7358Signed-off-by: Pekka Paalanen's avatarPekka Paalanen <pekka.paalanen@collabora.co.uk>
    Reviewed-by: Daniel Stone's avatarDaniel Stone <daniels@collabora.com>
    Reviewed-by: default avatarDerek Foreman <derekf@osg.samsung.com>
    Reviewed-by: Jonas Ådahl's avatarJonas Ådahl <jadahl@gmail.com>
    Reviewed-by: default avatarMartin Gräßlin <mgraesslin@kde.org>
    de24f4dd
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