From my limited experience bug hunting, calling #changed: from a thread other than the UI thread is a source of evil.�� There are too many assumptions throughout the system that the UI is single threaded.�� Can anyone advise me that is not a proper belief?

Then that implies that a Transcript implementation where #nextPut: direct calls #changed:

is not appropriate for use with multi-threaded applications.�� In Pharo, #changed: is only called from #stepGlobal, which is called from doOneCycle:. ��(This came about as a last minute bug fix before Pharo 3 release and maybe could use some cleanup.

Separating the UI from Transcript into its own viewer might be a good idea, but actually it would not solve Stef's case since his code would still be running in the UI thread -- unless the viewer ran in another thread, which would have its own complexities.

I think the point about efficiency is significant. The following example...

�� �� ��Time millisecondsToRun: [ 1000 timesRepeat: ��[ Transcript show: 'x' ] ]

on Squeak 4.5 -->��12749ms

on Pharo 50029 --> 2ms