I use the program "xfig" to exercise the bug. This program can be installed from the cygwin setup.exe program.
First case: ----------
I invoke X11 as "setsid XWin -multiwindow -clipboard &". I launch an xterm, and from there, I launch xfig. Within xfig, I select the tool for drawing rectangles. I launch the dialog box to select line color and fill color. While working in this dialog box, the X11 pointer will eventually disappear. I cannot do anything more at this point. I cannot change to focus to another X11 application. I cannot see the pointer in any X11 application. I have to kill XWin.
Second case: -----------
I invoke X11 as "setsid XWin -multiwindow &". I sleep for three seconds (my little trick for increasing stability). Then I do "setsid xwinclip &".
I launch an xterm and xfig as in the first case. I cause the X11 pointer to disappear as in the first case. Once again, all X11 applications are unusable. From a bash command window, I do a "kill -hup" on the running xwinclip process. This causes the X11 pointer to reappear. My X11 applications are now usable again.