property OnPressed: TNotifyEvent;
The OnPressed event occurs when the user clicks the the unpressed CaptionButton on title bar or selects in the SystemMenu the unchecked menu item associated with this button.
When user clicks the button which could be pressed (this controlled by AllowPress property), the button state will changed to down, Down property will be True and associated menu item in the system menu will be checked (if SystemMenu.ApplyToMenu is True).
AllowPress, Down, SystemMenu properties.
OnClick and OnReleased events.