void __fastcall UpdateControlState();
Respond to state change.
UpdateControlState is called automatically when any display change occurs that might affect the internal state of the control.
As implemented in TWinControl, UpdateControlState climbs the chain of parent controls to verify that each control in the chain has Showing equal to true. If this is the case, UpdateControlState makes sure that the Showing property is accurate for this control and for all its descendants. Descendant classes can extend this behavior.