UIHANDLER PreClick

Fires immediately before , (self, button, down)

Arguments

 * self : - Widget being clicked.
 * button : - "LeftButton", "RightButton", "MiddleButton", "Button4", "Button5", ..., "ButtonN"; however, virtual or overriden clicks  have aribtrary values using,  and.
 * down : - True when the button is pressed, false when it is released.

Details

 * Preceeded by and.
 * Blocked in the "up" direction by when it fires.
 * Blocked in the "up" direction after dragging with a button, or any time if the cursor is moved off the widget and not brought back before releasing the mouse button.