To reproduce: ./mach mochitest --disable-e10s layout/style/test/test_transitions_computed_values.html On my Windows machine, it crashes with just: thread '<unnamed>' panicked at 'index out of bounds: the len is 1 but the index is 1', ../src/libcore\slice.rs:524 and no further stack info...
I suspect this is also animation work related issue, so ni? hiro as well.
Interesting, I though I've never touched transition code, but will do. Oh the test file includes CSS animation test. sigh.
I did include a fix for this bug in a patch for bug 1330824.
Per comment 3, close this bug as FIXED.