FVWM: how to make button unpressed while moving off cursor from it

[for.register for.register]

Hi,

In fact,  I dont know how to say in in English,  Let me try:

For example , I want to close a window, I press and hold the left mouse
key on the "close button" . And now I dont want to close the window
so, I move off the cursor from the "close button", and release the mouse
key. And I hope the window is still there unclosed. How the configure this?

Thanks in advance.

Walter