I just set up a sample file to test this out and I find that all is working as expected.
I set the trigger script to fire OnObjectModify of the Type field.
Hmmm... You used a variation of my script above (minus the Halt Script step) and it worked??
I've only tested this on a Mac. Same for you? (Note that the field to which the trigger is attached is formatted as a drop-down list. But I believe the result was the same even when it was attached to a normal text-entry field. Also, other step directives work fine. The only one that's causing a problem is Go To Field.)
One last note... Probably irrelevant, but the Number field is next in the tab order to the trigger field (Type).
My script was identical to that which you posted. Five steps, no Halt.
It was tested on a Mac running 10.5.6.
The tab order of the fields was Type -> Number -> Date.
The Type field was formatted as a drop-down list.
I did see a note explaining the Halt step behaviour:
"When you execute the Halt Script script step with a "pre-event" trigger, Halt Script cancels the original event, similar to when False is returned from an Exit Script script step."
So if you used a pre-event trigger such as OnObjectSave or OnObjectExit, the script will cancel the event. I would need to work out what the implications might be. But it may be a clue.
In any event, it worked for me with OnObjectModify which is a post-event trigger.
I just tested it again and you're correct. It is now working with OnObjectModify (but not with OnObjectExit or OnObjectSave). Although I previously tried all three triggers many times and none of them worked, OnObjectModify works NOW -- so fixed is fixed.
I think you may be right and that OnObjectModify is the only post-event trigger of the three (although the names of the other two would imply that they're post rather than pre). I'm going to have to look at the explanations for script triggers more closely so I can get a solid grasp of which trigger is appropriate for which event, what constitutes an event, and the difference between pre and post events. In the meantime, thank you very much for testing this out.