The "Device" is somewhat useful as it allows you to draw widgets in GL windows. I think there is also an attempt to allow it to print, though I am unclear if it is actually possible to use it for that and if anybody has done so. I still feel the number of virtual functions on it is excessive.

It sounds like Cairo is nowhere near as slow as others have claimed. It would be great to get a real working implementation that can be used both by this and X11. I think all transformations have to be done by the calling code, as there is a need to adjust all paths for fill and stroke to remove antialiasing on horizontal and vertical lines, and allow 1-pixel lines to be usable. This indicates to me that transforms at least can be removed from the Device virtual functions.

Read through the libdecor code and it appears to be drawing everything directly. There is a "plugin", perhaps with the intention that the style of the window borders can change by changing the plugin, but I don't agree with this as the api to the plugin is going to swiftly grow to a baroque monstrosity, just like every other attempt to control the borders. There was a good reason Wayland wanted CSD. I personally do not see any problem with drawing the window borders ourselves, any complaint that they look different is IMHO not logical if the difference in appearance of every other widget is considered acceptable. Also not using this library will mean a single Wayland surface can be used, instead of two.