In that article last week I wrote how the sub-surfaces support was moved from being just a Weston compositor feature to now being part of core Wayland. Sub-surfaces allow for better support of applications like video players or windowed OpenGL content.
Pekka Paalanen, the developer at Collabora who originally spearheaded the Weston feature and then got it promoted to Wayland, has written a new blog post about sub-surfaces in Wayland for those interested in the low-level details.
Pekka summarized, "Sub-surfaces are intended for special cases, where you need to build a window from several buffers that are composited together, to make efficient use of the hardware resources. They are not meant for widgets in general, nor for pushing parts of application rendering to the compositor. Sub-surfaces are also not meant for things that are not integral parts of a window, like tooltips, menus, or drop-down boxes. These 'transient' surface types should be offered by the shell protocol."