In TinyCore we currently ship libx11 1.3.6 (the last version that builds without xcb).

It appears that gtk3 3.2.2 uses xlib from multiple threads, thus crashing with an xcb-less libx11. I was told at #gtk+ that things should still work without xcb.

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Output from gtk3-demo:

tc@box:~$ gtk3-demo 
Xlib: sequence lost (0x10223 > 0x2a3) in reply type 0x0!
.... a lot of xlib: sequence
Xlib: sequence lost (0x10000 > 0x2e3) in reply type 0x0!

(gtk3-demo:10234): Gdk-WARNING **: The program 'gtk3-demo' received an X Window System error.
This probably reflects a bug in the program.
The error was '60'.
  (Details: serial 26102 error_code 60 request_code 0 minor_code 26)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)