This won't work. For example: isOpposite('aa','dd') is true.

Yes, there is a lack of tests that catch problems when you start from the end. Some of the random can do it, but the chance is not that big.

I hope I will find time to go through the tests in all the languages soon. :-)

Thanks for taking the time to nudge me! :-)

The tests do not spot naive approach as in Stein256 solution (start from the end and choose first matching part).
Example call: isMerge("ioxo", "io", "xo") should return true.
Maybe a test like this could be added.