What text is in the field, and what kind of field it is? If more text is in the field than is visible you can move up/down in the field to see the additional content... maybe that is what you're seeing?
yes you are right here, but i have a condition to select the text for creating the comment on that text ,so that i can not make selectable property false.
If you want the text selectable property, then you should try wordwrap property and use multiline textfield instead of singleline. I think this will help you.