It should work if you use the MT Cog VM, that allows Squeak to  
continue running alongside primitives. Never tested it, though.

Quoting Tony Garnock-Jones <tonyg at ccs.neu.edu>:

> On 06/24/2013 09:28 AM, Juan Vuletich (mail lists) wrote:
>> I recently did something better fo Cuis. The idea is to let Morphic
>> handle that. If Morphic frame rate drops below some threshold, show the
>> wait cursor.
>
> Does this work also for long-running primitives? I think the  
> #showWhile: approach works in that situation.
>
> Tony

Cheers,
Juan Vuletich