Ancient Greek apostrophe marking elision

[James Kass via Unicode]

For U+2019, there's a note saying 'this is the preferred character to 
use for apostrophe'.

Mark Davis wrote,

 > When it is between letters it doesn't cause a word break, ...

Some applications don't seem to get that.  For instance, the 
spellchecker for Mozilla Thunderbird flags the string "aren" for 
correction in the word "aren’t", which suggests that users trying to use 
preferred characters may face uphill battles.