I believe this is usual behaviour of Textboxes, hence I hardly ever use them.
Have you tried to use a AutoText entry for your Textbox. Create a textbox
with your required settings, select all, Alt+F3, name it with a unique name.
Unfortuantely I am not sure if the box is floating outside the margin, if
AutoText will return exactly what you are after.
Are you using a Textbox purely for the insertion of a picture?
You don't need to do this unless the picture is to specifically outside the
usual margins, even then the Picture Layout options can acheive this very
easily.
Pictures can be inserted into the flow of text very easily, and without the
problems that a Textbox inherently brings, such as your description.
The picture can have various Layout settings which can produce the required
appearance you are after. Come back if you need further help.
DeanH