Text editor behaves incorrectly when inserting newlines

When inserting newlines halfway an existing line, an extra newline is added some characters further up, extra newlines are inserted

–
Some line of text here
–

Inserting a newline before 'here' would yield:

–
Some line of text

h

ere
–
or possibly
—
Some line of text

here
–

While I would expect it to result in
–
Some line of text

here
–
as it does properly in Firefox.

This behavior seems particularly present when the scrollbar on the right is available, but I've seen it happening as well with one or two line replies.

For those interested, here's the text I used when testing:
–
some line of text here

and some more here

htnesuoahcersabhtnse cuea ucesoa dcrsue dacuesantbeo bueoru eobru ewovudhec roud sedcures obtnsb c sdeo ucr. dens oubers. o ucers douetn derd u.bous .dcrs uetnosduegcedhoondu endu oec.

ecuou edo cedentueo uedotnd ucero d.conted u ore

oced rueotbebecue o

eodu erouc dueorc.dsoue
–

Me I'd be happy with an editor that didn't tell me how to format my messages with regard to newlines, much like the one used in the bug tracker.