If another message loop is started during scrolling, the WM_LBUTTONUP is retrieved by that message loop, and because an application does not have access to the scroll bar's internal message retrieval loop, WM_LBUTTONUP cannot be dispatched correctly. Scrolling is terminated once WM_LBUTTONUP is received. The task of this message loop is to keep track of scrolling and to send the appropriate scroll bar notification messages, WM_HSCROLL and WM_VSCROLL.
When scrolling, an internal message retrieval loop is started in Windows. This problem occurs usually when a message retrieval loop is executed as the result of actions taken for scrolling upon receiving one of the scroll bar notification messages. The type of scroll bar is irrelevant to this problem, that is, the same problem occurs regardless of whether the scroll bar is part of the window or is a scroll bar control. The scroll bar continuously scrolls even after you release the left mouse button.
The scroll bar continues to scroll after you release the mouse button Symptoms
