Im pretty sure the stop button is leftover from processing where it actually exits out of the little applet where graphics are shown. As far as i know, its never done anything with regard to arduino. I dont know how to stop compiling, but to stop upload (which takes much longer anyway) i just yank the usb cable (tho i dont reccommend it unless you have an avr programmer on hand in case you corrupt the bootloader). I havent dl'd 0018 yet, but id love to see a way to keep serial data in the window while still being able to shut off the serial port (for logging purposes). Thats a big reason i never use 0017 much.