Bug#1030909: Unable to run xscreensaver-demo or xscreensaver-settings

[Jamie Zawinski]

You may want to report a second bug against GTK, that calling XSynchronize (dpy, True) does not work -- it appears that GTK or GDK are setting that to false at some time later. 

Proof: Add this to, e.g., update_subproc_timer and run with --debug:
XSetWindowBackgroundPixmap (s->dpy, 0, 0);

A second or two after startup, this will cause an X error. Backtrace will show a stack that does not include update_subproc_timer or XSetWindowBackgroundPixmap.

Add XSynchronize (s->dpy, True); before that line. Now the backtrace is as expected.

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Jamie Zawinski • jwz.org • dnalounge.com