GRECLIPSE-1725. Eclipse hangs issue on Greclipse code assist scenario

Andy, think it's best if you have a look at this fix. Perhaps you could steer me in the right direction if this is not quite the fix you'd like to see 
I've reproduced this issue with a code snippet from GRECLIPSE-1695 that i fixed not too long ago... Pretty sure I've tested content assist while i was fixing it. Not sure what has changed recently aside for the introduction of MultiplexingCompletionParser. I've tried the same code snippet with JDT CompletionParser and result was still the same. BTW, I could reproduce this problem in 3.5 although i was fixing GRECLIPSE-1695 after 3.5 was released.