why the right-most text box of the Detail Section gets truncated,
Detail section of which?
Maybe because you're not allowing a horizontal scrollbar but it's needed (even though by your described dimensions it should not be)? Might be a combination of issues, like you are using MoveSize method as well and forgot that? Hmm, that's Déjà vu to me.
Also, you might not be seeing a form vertical scrollbar because there's no need for it but unless you actually disallow it, space is reserved for it. That could also be a problem.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.