got it:
in the hidden depths of the protected painters:
type
TmagiccxPCStandardPainter = class(TcxPCStandardPainter) end;
procedure TfrmSumGraphs.pcSUMDatabasesMouseDown(Sender: TObject;
Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
var
n: Integer;
tab: tcxTabSheet;
R: TRect;
pt: TPoint;
begin
n := pcSUMDatabases.IndexOfTabAt(X, Y);
if (n <> -1) then
begin
tab := pcSUMDatabases.Pages[n];
TmagiccxPCStandardPainter(pcSUMDatabases.Painter).StandardPainterPrepareOutTabImageAndTextData(tab.TabIndex);
R := TmagiccxPCStandardPainter(pcSUMDatabases.Painter).FOutTabImageAndTextData.TabImageRect;
if PtInRect(R, Point(X, Y)) then ShowMessage('It''s on the image !!!!');
end;
end;