Created attachment 90355 [details]
stacktrace of the issue, using GDK_SYNCHRONIZE=1

The issue has been reported on https://bugs.launchpad.net/ubuntu/+source/software-center/+bug/1211887

The stracktrace has been collected using GDK_SYNCHRONIZE=1 and b gdk_x_error

(the issue started in Ubuntu 13.10, webkit didn't change compared to Ubuntu 13.04 which didn't have the bug so it might be an issue with GTK or cairo...)

"...
#3  0xb6c9e81e in handle_response (dpy=dpy@entry=0x84f5e98, 
    response=0x93e1360, in_XReply=in_XReply@entry=1) at ../../src/xcb_io.c:325
        async = <optimized out>
        next = <optimized out>
#4  0xb6c9f888 in _XReply (dpy=dpy@entry=0x84f5e98, rep=rep@entry=0xbfffc8e0, 
    extra=extra@entry=0, discard=discard@entry=1) at ../../src/xcb_io.c:627
        event = <optimized out>
        req = 0x91eb330
        response = 0x98d20e8
        error = 0x0
        c = 0x84f67e0
        reply = <optimized out>
        current = 0x91eb330
        __PRETTY_FUNCTION__ = "_XReply"
#5  0xb6c9af5b in XSync (dpy=dpy@entry=0x84f5e98, discard=discard@entry=0)
    at ../../src/Sync.c:44
        rep = {type = 92 '\\', revertTo = 0 '\000', sequenceNumber = 0, 
          length = 110, focus = 119, pad1 = 3066699988, pad2 = 147884176, 
          pad3 = 3086534288, pad4 = 3065589449, pad5 = 3086532608}
#6  0xb6c9aff3 in _XSyncFunction (dpy=0x84f5e98) at ../../src/Synchro.c:35
No locals.
#7  0xb6c7c9ab in XCopyArea (dpy=0x84f5e98, src_drawable=85983698, 
    dst_drawable=85983515, gc=0x95f6c90, src_x=0, src_y=0, width=1, 
    height=200, dst_x=0, dst_y=0) at ../../src/CopyArea.c:61
        req = <optimized out>
#8  0xb6bd65de in _cairo_xlib_surface_put_shm (surface=surface@entry=0xa802900)
    at /build/buildd/cairo-1.12.16/src/cairo-xlib-surface-shm.c:1100
        rects = 0xbfffc9b0
        i = <optimized out>
..."