Got it fixed, only need one more line of code. Wasn't tracking which tiles needn't exploring properly. Great Kata as always.

getting a timeout now, so no more errors, thanks

it's been 3 years, I hardly doubt it lol

Haha, not yet! will do soon! Very busy days and your Kata is quite big. :)

This kata needs a performance version. That would be fun. :)

1: I agree, this is bad practice. However, I interpreted the "...you are given a two dimensional array..." combined with "...Your objective is to fill in the area..." as an authorisation to modify the input. It's a little ambiguous.
2: I have no clue, I could get prints from the random tests, but after that I get nothing. I ran the tests locally with "assert Arrays.deepEquals" and that did not appear to get stuck, so it might even be a bug in JUnit.

Please specify that the original input may not be altered. I've spent way too long on trying to debug why the hell the server exceeded the time limit, when the problem was me just filling in the original input.

mayhaps you should google what being a 'square' actually means.

hint: 216972900 is most certaintly square
2,3,5,7,11,13, etc. are most certaintly not ;)

Man I really wish codewars had Java 9, there are some stream improvements that I think could help. Will work on this kata soon :)