I'm not seeing any chopped message on the browser side:
Notice that the highlighted message in blue is 1658 bytes in length. Here is the same message received by mIRC as viewed in a /debug window:
Notice in the /debug window, it chops after ",375-37" whereas this clearly keeps going in the browser version (seen at the bottom of the first image).
This message is the one generated by typing /test with the example script I gave.