Am 23.07.2013 14:59, schrieb Jon TURNEY:
> I can't reproduce this.
>
> I think there must be some other factor at work: the XWin error message
> relates to a timeout during X -> Windows clipboard conversion, but I can't
> make clicking on a firefox bookmark cause that conversion to happen.
>
> There are definitely bugs in this area, so any additional information you can
> provide would be helpful.

After deeper investigation, it occurs that the problem is triggered by gnuplot in particular (I was effectively unable to 
reproduce it with a few other X11 applications, as xterm, xfig, xdvi or ddd). Most of the time I use gnuplot from within Octave, 
but the easiest way to reproduce the problem is by launching gnuplot directly in a Cygwin terminal window, then entering "plot 
sin(x), cos(x)" on the gnuplot prompt to open an X11 pop-up window containing a plot. Here is the terminal output from gnuplot:

$ gnuplot

          G N U P L O T
          Version 4.6 patchlevel 3    last modified 2013-04-12
          Build System: CYGWIN_NT-5.1 i686

          Copyright (C) 1986-1993, 1998, 2004, 2007-2013
          Thomas Williams, Colin Kelley and many others

          gnuplot home:     http://www.gnuplot.info
          faq, bugs, etc:   type "help FAQ"
          immediate help:   type "help"  (plot window: hit 'h')

Terminal type set to 'x11'
gnuplot> plot sin(x), cos(x)
gnuplot>

After this, clicking on Firefox bookmarks shows the problem - also when using the bookmark window opened with Ctrl-Shift-B. I 
also tried to relaunch FF in "safe mode" i.e. with add-ons disabled, with the same result. After closing gnuplot it is necessary 
to wait for a few minutes until FF's behaviour returns to normal; alternatively one can close the X server to restore the normal 
function of FF immediately.
The problem shows both under Windows XP (SP3) and under Windows 7 (SP 1). I attach the "cygcheck -s -v -r" output and the 
/var/log/xwin/XWin.0.log for the former platform. I don't know why most of the message lines in the XWin.0.log are duplicated 
without and with the [time stamp] in front (it is not the case under Windows 7) - but that's another problem.