Small backtrace howto.
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Before run i3 (startx) execute:
    ulimit -c unlimited

Use gdb and coredump file to make backtrace log:

    $ gdb /usr/bin/i3 core.i3.17555

    (gdb) set logging file backtrace.log
    (gdb) set logging on
    Copying output to backtrace.log.
    (gdb) backtrace full
    ...
    ...
    (gdb) set logging off
    Done logging to backtrace.log.
    (gdb) quit