DragDetect function (winuser.h)

Captures the mouse and tracks its movement until the user releases the left button, presses the ESC key, or moves the mouse outside the drag rectangle around the specified point. The width and height of the drag rectangle are specified by the SM_CXDRAG and SM_CYDRAG values returned by the GetSystemMetrics function.

BOOL DragDetect(
  [in] HWND  hwnd,
  [in] POINT pt
);

[in] hwnd

Type: HWND

A handle to the window receiving mouse input.

[in] pt

Type: POINT

Initial position of the mouse, in screen coordinates. The function determines the coordinates of the drag rectangle by using this point.

Type: BOOL

If the user moved the mouse outside of the drag rectangle while holding down the left button, the return value is nonzero.

If the user did not move the mouse outside of the drag rectangle while holding down the left button, the return value is zero.

The system metrics for the drag rectangle are configurable, allowing for larger or smaller drag rectangles.

Conceptual

GetSystemMetrics

Mouse Input

POINT

Reference