[Rd] stuck tcltk scrollbars under Windows XP

[John Fox]

Dear R-devel list members,

I've encountered a problem with my Rcmdr package under Windows XP and could 
use some advice:

The Rcmdr package uses the tcltk package to create menus and dialog boxes. 
My standard procedure when a dialog is created is to grab the focus -- e.g., by

     tkfocus(top)
     tkgrab(top)

(Here, top is a top-level window -- say, containing one or more 
scrollbars.) When the window is closed, I release the focus, destroy the 
window, and return the focus to another window -- e.g.,

     tkgrab.release(top)
     tkdestroy(top)
     . . .
     tkfocus(.commander)

(Here .commander is a top-level window residing in the global environment.)

This procedure works fine under Windows 2000 -- where I've done most of the 
testing of the Rcmdr package under Windows -- but I received a report of a 
problem from a Windows XP user, who noted that scrollbars tend to get 
stuck. I was able to reproduce the problem intermittently on an XP system, 
and found that the problem appears to go away when I remove the calls to 
tkgrab() and tkgrab.release().

You can test for the problem with the following code:

     window1 <- tktoplevel()
     window2 <- tktoplevel()
     xFrame <- tkframe(window2)
     xScroll <- tkscrollbar(xFrame, repeatinterval=5, command=function(...) 
tkyview(xBox, ...))
     xBox <- tklistbox(xFrame, height=4,
         selectmode="single", background="white", exportselection="FALSE",
         yscrollcommand=function(...) tkset(xScroll, ...))
     for (x in letters) tkinsert(xBox, "end", x)
     onOK <- function() {
         tkgrab.release(window2)
         tkfocus(window1)
         tkdestroy(window2)
         }
     OKbutton <- tkbutton(window2, text="OK", width="12", command=onOK, 
default="active")
     tkgrid(xBox, xScroll, sticky="nw")
     tkgrid.configure(xScroll, sticky="ns")
     tkgrid(xFrame, sticky="w")
     tkgrid(OKbutton, sticky="w")
     tkselection.set(xBox, 0)
     tkbind(window2, "<Return>", onOK)
     tkfocus(window2)
     tkgrab(window2)

I find that I can make the scrollbar stick under Windows XP by repeatedly 
and rapidly pressing the scroll-down button. As mentioned, removing the 
calls to tkgrab() and tkgrab.release() seems to eliminate the problem.

I wonder whether anyone else has encountered this problem or has any 
insight into its source. Am I doing something wrong here? I could simply 
remove the calls to tkgrab() and tkgrab.release() throughout the package, 
or make them optional, perhaps checking for Windows XP via shell("ver", 
intern=TRUE)[2], but I'd rather solve the problem.

I've also had a report of tk windows failing to stay on top of the R-GUI 
SDI Console in Windows XP, but I haven't been able to duplicate this 
problem, and it has never occurred in my testing under Windows 2000 and 
Linux. Has anyone else experienced this behaviour?


Thanks,
  John
-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox