Back in November I wrote that KWin was being ported to XCB and that with the KDE 4.10 feature window closing that this was a candidate for merging. Besides moving from Xlib to XCB, other KWin action items for the 4.11 window are AppMenu support, color correction work, performance improvements in compositing environments, improving window decorations, and more.
In a new blog post by KWin's Martin Gräßlin, he confirms this XCB porting work is ongoing. "With Christmas break over there is again quite some work happening for KWin. Of course given that 4.10 is close by a few bugs got fixed, but with master open for 4.11 we also have the first feature commits. Most of it is in the area of porting KWin to XCB. Those changes are not listed, but they are quite nice as each of them brings a small improvement due to the asynchronous nature of XCB."
XCB are the C bindings for X and has long been intended as a replacement to Xlib while reducing the size and complexity of the X library and providing direct access for tapping the X11 protocol. Over Xlib, XCB also allows for better multi-threading and a-synchronous support.
One of the main motivations for KWin finally transitioning over to XCB is that the Qt 5.0 tool-kit no longer uses Xlib. For Qt5 compatibility, XCB is mandated and thanks to improvements in the Linux graphics stack it will be possible to eventually bypass Xlib completely from KWin by using XCB and EGL.