Here's the code to my dialog. Notice that I initialize it with a long line (thats longer than the width of the scroll bar)
But it doesnt display the horizontal scroll bar to move to the right
I'm pretty sure its a bug...

dialog test {
title Test
size -1 -1 125 156
option dbu
list 2, 9 26 115 85, hsbar
}

on *:dialog:test:init:0: {
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo

}

You have to use the -z switch for /did to get it to update the scrollbar. I think the idea is so that you can add many lines, and then update after just the last one, which can make it work faster. Try like this:

on *:dialog:test:init:0: {
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -a test 2 /.MSG <> hi there wants your name dude my name is steveo
/did -az test 2 /.MSG <> hi there wants your name dude my name is steveo
}

(note the z on just the last /did)

edit: Maybe that should be made more clear in future versions of the help file. If I didn't happen to already know that, I would have thought it was a bug too.

There still is a but with the listbox hsbar.
After /did -z it updates too long. Instead of sizing to the end of the text it sizes to about 30% longer then the text displayed so there's a long white space at the end of the line, meaning that even if the hsbar isn't needed it still sizes way too long having the hsbar when it isn't needed.