Edit Control question

[ymmace]

I have an Edit Control in a Dialog . The "Style" of Property is "Child" for some specially usage.
I use the Edit Control to log some read data , once per line .

When the data are more than the showing area of the Edit Control , there will be a scroll bar .
But when I drag the scroll bar and move up the previous data , the data seem overlap .
like attached image below...
Why ??

I try to change the "Style" of Property to "Overlapped" , and the problem is solved.
But I must use "Child" Dialog Style.

Can Somebody help me~~~

Thx a lot...