Controls.TCustomControl.PaintWindow
From RAD Studio VCL Reference
Description
Renders the image of the control on a specified device context.
PaintWindow is called automatically when a custom control receives a WM_PAINT message. PaintWindow assigns the device context specified by the DC parameter to the custom control's canvas, calls the Paint method, and finally removes the device context. Override PaintWindow to provide additional responses to the WM_PAINT message. To draw the image of the control, override the Paint method instead.