I assume this problem comes into action when you use a bitmap for buffering. I also had to fix the paint function like this:
{$IFDEF LCLCocoa}
StretchBlt(canvas.handle,0,0,Width,Height,FBuffer.Canvas.Handle,0,0,
FBuffer.Width, FBuffer.Height, SRCCOPY);
{$ELSE}
canvas.draw(0, 0, FBuffer);
{$ENDIF}
and of course in the uses clause there has to be a:
uses
{$ifdef LCLCocoa}
CocoaGDIObjects, MacOSAll,
{$endif}
Christian