Responsivenes is a per-client thing so we move the ping/pong functionality
to xdg_shell. Having this per-window was carries over from the EWMH
protocol, where the WM has no other way to do this. In wayland, the
compositor can directly ping the client that owns the surface.
This is used to figure out the size of "invisible" decorations, which we'll
use to better know the visible extents of the surface, which we can use for
constraining, titlebars, and more.
This is equivalent to WM_DELETE_WINDOW request under X11, or equivalent
to pressing the "close" button under CSD. Weston currently doesn't have
a compositor-side way to close the window, so no new code is needed on
its side.
xdg_shell is a protocol aimed to substitute wl_shell in the long term,
but will not be part of the wayland core protocol. It starts as a
non-stable API, aimed to be used as a development place at first, and
once features are defined as required by several desktop shells, we can
finally make it stable.
It provides mainly two new interfaces: xdg_surface and xdg_popup.
The xdg_surface interface implements a desktop-style window, that can be
moved, resized, maximized, etc. It provides a request for creating
child/parent relationship, called xdg_surface.set_transient_for.
The xdg_popup interface implements a desktop-style popup/menu. A
xdg_popup is always transient for another surface, and also has implicit
grab.
