No need in Update/Modified calls actually.

tutorials/cocoa/grad.C

@@ -84,6 +84,4 @@ void grad()
    hist->SetFillColor(histGradient);
    hist->FillRandom("gaus", 100000);
    hist->Draw();
-
-   cnv->Update();
 }

tutorials/cocoa/gradients.C

@@ -107,7 +107,4 @@ void gradients()
    //
    pie->SetRadius(0.4);
    pie->Draw("rsc");
-   
-   c->Modified();
-   c->Update();
 }