text isn't staying in the text box?

[seeny]

it's hard to explain but if i go into some webpage like macrumors and try to post a reply i get the txt box as normal, but after i press return to start a new line the text gets bigger, if i continue to the bottom of the text box instead of scrolling down the text either disapears or goes out of the text box, this only seems to have happened since ios4, it's happening on my new iphone 4 and my 3g since updating to io4, is there a fix

i've tried resetting but it didn't do anything

 

[seeny]

update

if i type it up first in notes and then paste the txt in it's ok

i've tried this on 2 other iphone 4's and it's the same on them