Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!cs.utexas.edu!husc6!cmcl2!rocky8!cucard!ccnysci!alexis
From: alexis@ccnysci.UUCP (Alexis Rosen)
Newsgroups: comp.sys.mac.programmer
Subject: Odd scrollbar behavior with TrackControl
Message-ID: <942@ccnysci.UUCP>
Date: 22 Oct 88 06:23:11 GMT
Reply-To: alexis@ccnysci.UUCP (Alexis Rosen)
Lines: 51


In the last week I built a few utilities to make my life easier. They are not
real complicated- they don't have event loops, for example. I didn't expect
much trouble, and indeed for the most part everything went smoothly. There
was one big problem, however. For the help screen I put up, I wanted a working
scrollbar. So I built a DLOG & DITL & CNTL, and that seemed to work okay.
Then I wrote this bit of code to handle thumb dragging (before I did the
ActionProc for the scroll arrows):
WHILE iNo <> 1 DO BEGIN
 ModalDialog(NIL, iNo);
 IF iNo = 2 THEN BEGIN
   TEScroll(0, -tx^^.lineHeight * (GetCtlValue(ControlHandle(item)) - pos),tx);
   pos := GetCtlValue(ControlHandle(item))
   END
 END

(BTW, this is in LSP 1.11)

Everything still worked fine. Except- When I dragged the thumb, it didn't
drag a grey outline of the thumb the way it ordinarily does. It drew a two
pixel thick box that was inset one pixel from the regular thumb, and dragged
that around. This was weird, but it didn't bother me much.

What did bother me was the crash I kept getting when I defined an ActionProc
for the scrollbar control. I said:
  SetCtlAction(ControlHandle(item), @Scroller);
just before starting the ModalDialog loop. Scroller is a procedure that handles
clicks in the arrows and grey regions. The wierd thing is that the ActionProc
worked just fine. It did everything it was supposd to. BUT- now, the instant
I clicked on the thumb, the machine would crash. The debugger showed me that
when I was clicking in the thumb, my ActionProc was passed a bad ControlHandle.
Clicks elsewhere passed the correct handle.

This was so annoying that I skipped ModalDialog and did everything in a
window. No luck. Calling FindControl and TrackControl myself didn't improve
things at all. In the end, I brute-forced it by saying
IF TestControl(scrl, evnt.where) = inThumb THEN
  i := TrackControl(ctrl, evnt.where, NIL)
ELSE
  i := TrackControl(ctrl, evnt.where, Pointer(-1));

which means that if it's in the thumb, don't do the ActionProc, otherwise
go do it.

Does anyone have any idea what's going on???

----
Alexis Rosen                       alexis@dasys1.UUCP  or  alexis@ccnysci.UUCP
Writing from                       {allegra,philabs,cmcl2}!phri\
The Big Electric Cat                                       uunet!dasys1!alexis
Public UNIX                           {portal,well,sun}!hoptoad/