As far as I can tell now, this issue has a relation to the compositor:
- it occurs when selected rendering backend OpenGl 3.1, interface EGL
- it occurs when selected rendering backend OpenGl 2.0, interface EGL

- it does not occur when selected interface GLX
- it does not occur when selected XRender

As it may be OpenGl related, it will be useful to know that that this is on Intel Haswell:

# kwin_x11 --replace&                                                                                                                                                                                  
ferry@chromium:~$ QXcbConnection: XCB error: 8 (BadMatch), sequence: 1434, resource id: 27262982, major code: 42 (SetInputFocus), minor code: 0                                                 
OpenGL vendor string:                   Intel Open Source Technology Center                                                                                                                     
OpenGL renderer string:                 Mesa DRI Intel(R) Haswell Mobile                                                                                                                        
OpenGL version string:                  3.0 Mesa 11.0.2                                                                                                                                         
OpenGL shading language version string: 1.30                                                                                                                                                    
Driver:                                 Intel                                                                                                                                                   
GPU class:                              Haswell                                                                                                                                                 
OpenGL version:                         3.0                                                                                                                                                     
GLSL version:                           1.30                                                                                                                                                    
Mesa version:                           11.0.2                                                                                                                                                  
X server version:                       1.17.2                                                                                                                                                  
Linux kernel version:                   4.3                                                                                                                                                     
Requires strict binding:                yes                                                                                                                                                     
GLSL shaders:                           yes                                                                                                                                                     
Texture NPOT support:                   yes                                                                                                                                                     
Virtual Machine:                        no