Why am I not receiving MouseLeave messages in a Control in my Form?

This usually happens when a Control doesn’t know that the mouse has left its bounds following a mouse enter. This results in the Control not throwing a MouseLeave event for subsequent mouse moves over it.

This is possible if you performed some operation when the mouse was within a Control that made it lose its mouse capture, for example, opening a top-level window over the Control such that the mouse is now over this new window.

To work around this problem send a WM_MOUSELEAVE manually to the original Control before the offending operation.

class NativeMethods
{
	[DllImport('user32.dll', CharSet=CharSet.Auto)]
	extern public static IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, IntPtr lParam)  ;
}

// Send a WM_MOUSELEAVE manually to a window.
NativeMethods.SendMessage(control.Handle, NativeMethods.WM_MOUSELEAVE, IntPtr.Zero, IntPtr.Zero);