TWinControl.OnEnter
From OpenEUO
EV OnEnter Occurs when a control receives the input focus.
Use the OnEnter event handler to cause any special processing to occur when a control becomes active. The OnEnter event does not occur when switching between forms or between another application and the application that includes the control. When switching between controls in separate container controls such as the TPanel and the TGroupBox controls, an OnEnter event occurs for the container before the OnEnter event of the contained control.
