Thoughts about decoration information in the xdg_shell

[Martin Gräßlin]

Hi all,

this is a reply to the topic of whether to have information about decorations 
in the xdg_shell at all (see [1]). Sorry that I don't reply to the right 
message, had not been subscribed to the list.

I want to outline why we in the KDE Plasma team would prefer to have the 
possibility to tell clients whether to draw decorations or not. In Plasma we 
have multiple shells and will be able to switch at runtime. From the window 
manager perspective the main difference is currently the handling of window 
decorations. Our touch oriented shell doesn't have decorated windows, our 
netbook shell only has decorated windows for dialogs and our desktop shell has 
a typical window decorated style, but not enforcing SSD (there are valid use 
cases like yakuake).

This gives us quite some flexibility to adjust at runtime (add keyboard to 
tablet to get a desktop). An application doesn't need to know what the shell 
currently looks like. The shell takes care of handling all of that.

If we don't have information about whether the window should be decorated or 
not in the xdg_shell we would need to adjust all applications to know about 
our various shells. Otherwise a consistent user experience would not be 
possible. Also it removes all possibilities to take this further in the 
future.

Now I don't want to force SSD to anybody nor do I want to be forced to CSD. 
Therefore I want to have a very simple spec, which allows the compositor to 
tell the clients whether they are expected to render decos themselves (enforce 
CSD, e.g. GNOME Shell), whether the compositor doesn't care, but prefer them 
to not render themselves (e.g. Plasma Desktop case) or whether the compositor 
wants the client to not render any handles at all (enforce SSD, Plasma Active 
case).

In addition the client should tell the compositor whether it draws handles or 
not. So that the SSD-supporting compositor doesn't add a decoration around a 
CSD client.

I think this would be a good solution. It would require clients to support 
CSD, but also to not render them in shells, which prefer to not have them. 
Also it doesn't force CSD applications to support it. They could just announce 
that they do CSD no matter what.

Opinions? Would that be acceptable for everybody?

Cheers
Martin Gräßlin

[1] http://lists.freedesktop.org/archives/wayland-devel/2013-November/011980.html
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