That may be the gist of the problem, but the problem states 'The string has a length greater or equal to one and contains only letters from a to z.' Adding extra error handling for other symbols and capital letters is not optimal as it's assumed this is already handled before the string is passed in.

Providing invalid characters to the input string is what the problem is all about

Not an issue. The instruction is clear.
If the mouse is out of screen, return double[] { 0, 0 }

Code template updated.

Fixed, thanks!

There are capital letters in the PHP test case, presumably unintended?

Fair enough. I've done enough real-world coding to never trust an error condition to be 'valid' for a particular state, however ;)

Good idea but the description says:

The string ... contains only letters from ato z

I've had that - even submitting a 'blank' response (i.e. return null;) will cause this. Do any of the test cases start with a zero? Or go beyond the bounds of a standard int?