Jira version : v4.2.2-b589#589
Navigator : Google chrome 9.0.597.98

Scenario :
1- take a issue
2- clic on button "comment" (bottom of the page)
3- write 3 lines
4- clic on the second line (cursor moves on the second line)
5- clic at the end (middle of the text box for example)

=> cursor moves at the beginning of the last line and not at the and (as expected)

Thanks & Regards,
Charles