Created attachment 48692 [details] gdb bt File -> Create -> Screenshot -> Snap -> gimp-2.7: cairo-surface.c:1189: cairo_surface_mark_dirty_rectangle: Assertion `! _cairo_surface_has_snapshots (surface)' failed. Reproducible with gimp git commit ab3fe10... (no crash with git master) & cairo-git master, no crash when cairo built without --enable-xlib-xcb.
Created attachment 48694 [details] gdb bt full
Since you mention that it works with gimp git master, I looked at the recent gimp commits. This is what I found: http://git.gnome.org/browse/gimp/commit/?id=3efa2062c517e3365efc1f1f7a22a0016a85f35c https://bugzilla.gnome.org/show_bug.cgi?id=650426 I'd say that this is indeed a gimp bug and the xlib backend just didn't cause a snapshot to be created.
On the face of it, that's actually indicative of a bug in GIMP. It is either using cairo_surface_mark_dirty() when there is no need (it is not bypassing cairo and touching the surface directly), or it is bypassing cairo and has not called cairo_surface_flush(). The alternative, is of course, that the xlib-xcb hookup is not complete and the flush is not being propagated to the xcb surface...
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