George,
We could replicate the reported problem at our end.The dev team is looking into fixing it for the next update (we just released CodeMix 3). We will keep you posted about the fix schedule.
As a workaround, you can paste the huge content in a separate editor and then open the file in the CodeMix’s JavaScript editor to continue working.
Apologies for inconvenience caused. Please let us know if you see any other issues with CodeMix.
–Swapna
Genuitec Support