WinForms TextBox horizontal scrollbar jumps to rightmost edge when clicked/dropped

Question

I have a simple WinForm application that involves a TextBox displaying multiple lines of... get this... text. When I click on the horizontal scrollbar on this multiline TextBox, the horizontal scrollbar jumps all the way to the right, which I really don't want it to do. It will cause pretty serious usability issues.

Anyone know what causes this and/or how to fix it?

EDIT: I apologize. This only happens when I click on the thumb of the horizontal scroll bar. I should have been clearer.

Targeting .NET Framework 3.5, Running .NET Framework 4.0/Visual Studio C# Express 2010 on WinXP