The bug

Number of characters before line cutoff in chat is not same when unicode mode is enabled and when it is off.

How to reproduce

1. Enable unicode
2. Run the following command (to put the maximum length of text on screen):

   /tellraw @p {"text":"12345 12345 12345 12345 12345 12345 12345 12345 12345 12345 12345 12345 12345"}

3. Disable unicode
4. Run the command again (to ensure that the two messages have the same number of characters)
   → Notice that the cutoffs are not the same in the identical messages

I think that the best way to fix this problem is to increase length of text-line when unicode-mode is disabled.

Code analysis

Code analysis by __null can be found in this comment.