I'm far away from being an expert for such types of problems, but I suspect that it has something to do with the xserver's built-in screensaver as this happens exactly after 10 minutes idle time, the xserver's default blanking time. Another hint that this value is not controled by ScreenShield is the fact, that while I'm watching video the screen is going blank after 10 minutes without keybord/mouse events, too.

After further testing I can add: xset s off helps. So ScreenShield seems not to disable xserver' own screensaver, causing the above mentioned troubles.

Just for the record: while xset s off helped at some stage in 3.5.90 cycle, for final release and current git master it does not. Shell keeps dying with X error when the screen blanks.

Forget about my last comment. After rechecking it, I found out that cycle was still set to a non-zero value. So xset s 0 0 && xset s off makes sure that this does not happen again. BTW, this is similar to that what gnome-screensaver is doing in src/gs-watcher-x11.c.

Created attachment 222485 [details] [review]
ScreenShield: Disable X ScreenSaver extension

The ScreenSaver extension is so broken that it be configured to prevent
drawing when the screensaver is active, with a BadDrawable. Protect ourselves
from that by disabling it.

Could you try the attached patch and confirm if the bug is fixed?
Thank you!

The good news is, that with the patch the shell does not crash, but
a first quick test shows, that the shell now fails to blank/lock the screen. Within the next few days I'll have a bit more time to test it.

I looked over your patch and found the following:

>  /* Turn off the server builtin saver if it is now running. */
>  XForceScreenSaver (GDK_DISPLAY_XDISPLAY (gdk_display_get_default ()), ScreenSaverReset);

It's true that it is in the original code, but there it is executed conditionally and not every time you call the function. For testing purposes I simply removed it, and blanking/locking was working again. If at all, maybe XForceScreenSaver should only be called at startup? (I don't know if this is the correct approach).

To confirm that the patch is working (when testing without the XForceScreensaver call it does currently, so many thanks), I'll need to test it a bit more as it sometimes happend, that even when I turned off xservers' screensaver (I did it only once in the session), gnome-shell recieved the X error when screen was turning off after blanking/locking. But here we are calling it right before locking again, maybe that does the trick.

Lets attach another trace, this time I've set a breakpoint at gdk_x_error and installed more debugging packages.

GNU gdb (GDB) 7.4.1-debian
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i486-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
attach: Datei oder Verzeichnis nicht gefunden.
Attaching to process 16263
Reading symbols from /usr/bin/gnome-shell...Reading symbols from /usr/lib/debug/usr/bin/gnome-shell...done.
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Reading symbols from /usr/lib/gnome-shell/libgnome-shell.so...Reading symbols from /usr/lib/debug/usr/lib/gnome-shell/libgnome-shell.so...done.
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Loaded symbols for /usr/lib/gnome-shell/libgnome-shell.so
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[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/i686/cmov/libthread_db.so.1".
[New Thread 0xa05d4b70 (LWP 16385)]
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Loaded symbols for /usr/lib/libgnome-bluetooth.so.11
Reading symbols from /usr/lib/i386-linux-gnu/libpangoft2-1.0.so.0...Reading symbols from /usr/lib/debug/usr/lib/i386-linux-gnu/libpangoft2-1.0.so.0.3100.0...done.
done.
Loaded symbols for /usr/lib/i386-linux-gnu/libpangoft2-1.0.so.0
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Loaded symbols for /usr/lib/i386-linux-gnu/libfreetype.so.6
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Reading symbols from /lib/i386-linux-gnu/i686/cmov/libresolv.so.2...Reading symbols from /usr/lib/debug/lib/i386-linux-gnu/i686/cmov/libresolv-2.13.so...done.
done.
Loaded symbols for /lib/i386-linux-gnu/i686/cmov/libresolv.so.2
Reading symbols from /lib/i386-linux-gnu/libpng12.so.0...(no debugging symbols found)...done.
Loaded symbols for /lib/i386-linux-gnu/libpng12.so.0
Reading symbols from /usr/lib/i386-linux-gnu/libpixman-1.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libpixman-1.so.0
Reading symbols from /usr/lib/i386-linux-gnu/libxcb-shm.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libxcb-shm.so.0
Reading symbols from /usr/lib/i386-linux-gnu/libxcb-render.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libxcb-render.so.0
Reading symbols from /usr/lib/i386-linux-gnu/pulseaudio/libpulsecommon-2.1.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/pulseaudio/libpulsecommon-2.1.so
Reading symbols from /lib/i386-linux-gnu/libcap.so.2...(no debugging symbols found)...done.
Loaded symbols for /lib/i386-linux-gnu/libcap.so.2
Reading symbols from /usr/lib/i386-linux-gnu/libjson.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libjson.so.0
Reading symbols from /lib/i386-linux-gnu/libpcre.so.3...(no debugging symbols found)...done.
Loaded symbols for /lib/i386-linux-gnu/libpcre.so.3
Reading symbols from /usr/lib/i386-linux-gnu/libXau.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libXau.so.6
Reading symbols from /usr/lib/i386-linux-gnu/libXdmcp.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libXdmcp.so.6
Reading symbols from /lib/i386-linux-gnu/i686/cmov/libcrypt.so.1...Reading symbols from /usr/lib/debug/lib/i386-linux-gnu/i686/cmov/libcrypt-2.13.so...done.
done.
Loaded symbols for /lib/i386-linux-gnu/i686/cmov/libcrypt.so.1
Reading symbols from /lib/i386-linux-gnu/libgpg-error.so.0...(no debugging symbols found)...done.
Loaded symbols for /lib/i386-linux-gnu/libgpg-error.so.0
Reading symbols from /usr/lib/i386-linux-gnu/libtasn1.so.3...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libtasn1.so.3
Reading symbols from /lib/i386-linux-gnu/libudev.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/i386-linux-gnu/libudev.so.1
Reading symbols from /usr/lib/i386-linux-gnu/libharfbuzz.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libharfbuzz.so.0
Reading symbols from /usr/lib/i386-linux-gnu/libXtst.so.6...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libXtst.so.6
Reading symbols from /lib/i386-linux-gnu/libwrap.so.0...(no debugging symbols found)...done.
Loaded symbols for /lib/i386-linux-gnu/libwrap.so.0
Reading symbols from /usr/lib/i386-linux-gnu/libsndfile.so.1...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libsndfile.so.1
Reading symbols from /usr/lib/i386-linux-gnu/libasyncns.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libasyncns.so.0
Reading symbols from /lib/i386-linux-gnu/libattr.so.1...(no debugging symbols found)...done.
Loaded symbols for /lib/i386-linux-gnu/libattr.so.1
Reading symbols from /usr/lib/libgraphite2.so.2.0.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/libgraphite2.so.2.0.0
Reading symbols from /lib/i386-linux-gnu/i686/cmov/libnsl.so.1...Reading symbols from /usr/lib/debug/lib/i386-linux-gnu/i686/cmov/libnsl-2.13.so...done.
done.
Loaded symbols for /lib/i386-linux-gnu/i686/cmov/libnsl.so.1
Reading symbols from /usr/lib/i386-linux-gnu/libFLAC.so.8...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libFLAC.so.8
Reading symbols from /usr/lib/i386-linux-gnu/libvorbisenc.so.2...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libvorbisenc.so.2
Reading symbols from /lib/i386-linux-gnu/i686/cmov/libnss_compat.so.2...Reading symbols from /usr/lib/debug/lib/i386-linux-gnu/i686/cmov/libnss_compat-2.13.so...done.
done.
Loaded symbols for /lib/i386-linux-gnu/i686/cmov/libnss_compat.so.2
Reading symbols from /lib/i386-linux-gnu/i686/cmov/libnss_nis.so.2...Reading symbols from /usr/lib/debug/lib/i386-linux-gnu/i686/cmov/libnss_nis-2.13.so...done.
done.
Loaded symbols for /lib/i386-linux-gnu/i686/cmov/libnss_nis.so.2
Reading symbols from /lib/i386-linux-gnu/i686/cmov/libnss_files.so.2...Reading symbols from /usr/lib/debug/lib/i386-linux-gnu/i686/cmov/libnss_files-2.13.so...done.
done.
Loaded symbols for /lib/i386-linux-gnu/i686/cmov/libnss_files.so.2
Reading symbols from /usr/lib/gtk-3.0/3.0.0/theming-engines/libadwaita.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/gtk-3.0/3.0.0/theming-engines/libadwaita.so
Reading symbols from /usr/lib/i386-linux-gnu/librsvg-2.so.2...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/librsvg-2.so.2
Reading symbols from /usr/lib/i386-linux-gnu/gtk-3.0/modules/libpk-gtk-module.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/gtk-3.0/modules/libpk-gtk-module.so
Reading symbols from /usr/lib/i386-linux-gnu/gtk-3.0/modules/libcanberra-gtk-module.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/gtk-3.0/modules/libcanberra-gtk-module.so
Reading symbols from /usr/lib/i386-linux-gnu/dri/i915_dri.so...Reading symbols from /usr/lib/debug/usr/lib/i386-linux-gnu/dri/i915_dri.so...done.
done.
Loaded symbols for /usr/lib/i386-linux-gnu/dri/i915_dri.so
Reading symbols from /usr/lib/i386-linux-gnu/libdricore8.1.0.so.1...Reading symbols from /usr/lib/debug/usr/lib/i386-linux-gnu/libdricore8.1.0.so.1.0.0...done.
done.
Loaded symbols for /usr/lib/i386-linux-gnu/libdricore8.1.0.so.1
Reading symbols from /usr/lib/i386-linux-gnu/libdrm_intel.so.1...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libdrm_intel.so.1
Reading symbols from /usr/lib/i386-linux-gnu/libpciaccess.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libpciaccess.so.0
Reading symbols from /usr/lib/i386-linux-gnu/gio/modules/libdconfsettings.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/gio/modules/libdconfsettings.so
Reading symbols from /usr/lib/i386-linux-gnu/libcanberra-0.29/libcanberra-pulse.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libcanberra-0.29/libcanberra-pulse.so
Reading symbols from /usr/lib/gjs-1.0/langNative.so...Reading symbols from /usr/lib/debug/usr/lib/gjs-1.0/langNative.so...done.
done.
Loaded symbols for /usr/lib/gjs-1.0/langNative.so
Reading symbols from /usr/lib/gjs-1.0/gettextNative.so...Reading symbols from /usr/lib/debug/usr/lib/gjs-1.0/gettextNative.so...done.
done.
Loaded symbols for /usr/lib/gjs-1.0/gettextNative.so
Reading symbols from /usr/lib/gjs-1.0/formatNative.so...Reading symbols from /usr/lib/debug/usr/lib/gjs-1.0/formatNative.so...done.
done.
Loaded symbols for /usr/lib/gjs-1.0/formatNative.so
Reading symbols from /usr/lib/gjs-1.0/mainloop.so...Reading symbols from /usr/lib/debug/usr/lib/gjs-1.0/mainloop.so...done.
done.
Loaded symbols for /usr/lib/gjs-1.0/mainloop.so
Reading symbols from /usr/lib/gjs-1.0/cairoNative.so...Reading symbols from /usr/lib/debug/usr/lib/gjs-1.0/cairoNative.so...done.
done.
Loaded symbols for /usr/lib/gjs-1.0/cairoNative.so
Reading symbols from /usr/lib/gjs-1.0/dbusNative.so...Reading symbols from /usr/lib/debug/usr/lib/gjs-1.0/dbusNative.so...done.
done.
Loaded symbols for /usr/lib/gjs-1.0/dbusNative.so
Reading symbols from /usr/lib/libgjs-dbus.so.0...Reading symbols from /usr/lib/debug/usr/lib/libgjs-dbus.so.0.0.0...done.
done.
Loaded symbols for /usr/lib/libgjs-dbus.so.0
Reading symbols from /usr/lib/gjs-1.0/system.so...Reading symbols from /usr/lib/debug/usr/lib/gjs-1.0/system.so...done.
done.
Loaded symbols for /usr/lib/gjs-1.0/system.so
Reading symbols from /usr/lib/i386-linux-gnu/libibus-1.0.so.5...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libibus-1.0.so.5
Reading symbols from /usr/lib/i386-linux-gnu/gio/modules/libgioremote-volume-monitor.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/gio/modules/libgioremote-volume-monitor.so
Reading symbols from /usr/lib/i386-linux-gnu/libgvfscommon.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libgvfscommon.so.0
Reading symbols from /lib/i386-linux-gnu/i686/cmov/libutil.so.1...Reading symbols from /usr/lib/debug/lib/i386-linux-gnu/i686/cmov/libutil-2.13.so...done.
done.
Loaded symbols for /lib/i386-linux-gnu/i686/cmov/libutil.so.1
Reading symbols from /usr/lib/i386-linux-gnu/libbluray.so.1...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libbluray.so.1
Reading symbols from /usr/lib/i386-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-svg.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-svg.so
Reading symbols from /usr/lib/i386-linux-gnu/pkcs11/gnome-keyring-pkcs11.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/pkcs11/gnome-keyring-pkcs11.so
Reading symbols from /usr/lib/libaccountsservice.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/libaccountsservice.so.0
Reading symbols from /usr/lib/libupower-glib.so.1...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/libupower-glib.so.1
Reading symbols from /usr/lib/libgdm.so.1...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/libgdm.so.1
Reading symbols from /usr/lib/i386-linux-gnu/pango/1.8.0/modules/pango-basic-fc.so...Reading symbols from /usr/lib/debug/usr/lib/i386-linux-gnu/pango/1.8.0/modules/pango-basic-fc.so...done.
done.
Loaded symbols for /usr/lib/i386-linux-gnu/pango/1.8.0/modules/pango-basic-fc.so
Reading symbols from /usr/lib/i386-linux-gnu/gio/modules/libgiognutls.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/gio/modules/libgiognutls.so
Reading symbols from /usr/lib/libjavascriptcoregtk-3.0.so.0...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/libjavascriptcoregtk-3.0.so.0
Reading symbols from /usr/lib/i386-linux-gnu/libicui18n.so.48...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libicui18n.so.48
Reading symbols from /usr/lib/i386-linux-gnu/libicuuc.so.48...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libicuuc.so.48
Reading symbols from /usr/lib/i386-linux-gnu/libicudata.so.48...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/libicudata.so.48
Reading symbols from /usr/lib/i386-linux-gnu/gio/modules/libgiognomeproxy.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/gio/modules/libgiognomeproxy.so
Reading symbols from /usr/lib/i386-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so...(no debugging symbols found)...done.
Loaded symbols for /usr/lib/i386-linux-gnu/gdk-pixbuf-2.0/2.10.0/loaders/libpixbufloader-png.so
Reading symbols from /usr/lib/i386-linux-gnu/gconv/ISO8859-1.so...Reading symbols from /usr/lib/debug/usr/lib/i386-linux-gnu/gconv/ISO8859-1.so...done.
done.
Loaded symbols for /usr/lib/i386-linux-gnu/gconv/ISO8859-1.so
0xb76fd430 in __kernel_vsyscall ()
(gdb) break gdk_x_error
Breakpoint 1 at 0xb5ed8160: file /build/gtk+3.0-3.5.12.81-9d3c81/gdk/x11/gdkmain-x11.c, line 268.
(gdb) continue
Continuing.
[New Thread 0xa0dd5b70 (LWP 16572)]
[Thread 0xa0dd5b70 (LWP 16572) exited]

Breakpoint 1, gdk_x_error (xdisplay=0x9482470, error=0xbfd93e60)
    at /build/gtk+3.0-3.5.12.81-9d3c81/gdk/x11/gdkmain-x11.c:268
268	{
(gdb) t a a bt

+ Trace 230773

$1 = {void (void)} 0xb75180e0 <gjs_dumpstack>
(gdb) quit
A debugging session is active.

Sorry for the above message, I forgot to strip the header.

Good news, I think I finally found the cause for the X window system error, and - surprise - it is not related to gnome-shell but to a buggy combination of the intel driver running on a machine using kernel 3.5.

Anyway, the built-in screensaver should be suspended, otherwise it may happen, that while whatching videos the screen goes blank.

Your patch prevents that, if you modify it a bit to call XForceScreenSaver () only at initialisation and not when locking, too, as this will prevent screen locking in general.

Review of attachment 222485 [details] [review]:

Here the reference to the BadDrawable may be wrong

Review of attachment 222485 [details] [review]:

::: js/ui/screenShield.js
@@ +362,3 @@
+        // a BadDrawable error
+        // We do it again when about to lock, to protect us from xset or xlock
+        Shell.util_disable_x_screensaver();

If you make the call to XForceScreenSaver conditional, here it would be good to have: 

Shell.util_disable_x_screensaver(true);

Drop the comment with the BadDrawable error as it seems not to be true

@@ +429,3 @@
     _onStatusChanged: function(status) {
         if (status == GnomeSession.PresenceStatus.IDLE) {
+            Shell.util_disable_x_screensaver();

Here we'll need:
Shell.util_disable_x_screensaver(false);

::: src/shell-util.c
@@ +722,3 @@
+/* Shamelessly stolen from gnome-screensaver gs-watcher-x11.c */
+void
+shell_util_disable_x_screensaver (void)

Better:
shell_util_disable_x_screensaver (gboolean force)

@@ +770,3 @@
+
+  /* Turn off the server builtin saver if it is now running. */
+  XForceScreenSaver (GDK_DISPLAY_XDISPLAY (gdk_display_get_default ()), ScreenSaverReset);

Better:
if (force)
  XForceScreenSaver (GDK_DISPLAY_XDISPLAY (gdk_display_get_default ()), ScreenSaverReset);

::: src/shell-util.h
@@ +48,3 @@
                                                int                rowstride);
 
+void     shell_util_disable_x_screensaver      (void);

Better:
void shell_util_disable_x_screensaver (gboolean force);

Sorry, but I don't understand why the call to XForceScreenSaver should be conditional. The shell doesn't use the X ScreenSaver extension, so any use of xlock, xset or any other commands that rely on it would conflict with built-in screen locking.

It should be conditional (as it is in gs-watcher-x11.c) because when using your patch, the screen is never locking, no matter what is set via gnome-control-center/dconf (if it is for you, I don't know what is going wrong here).It only works when XForceScreensaver is not called in _onStatusChanged, or said better, at least then and only then it works here. 

A working possibility to omit the bool value is to call Shell.util_disable_x_screensaver() later in _onStatusChanged, after the general setup is done, but if this is a correct approach, you're more experienced to decide than I am.

Why not to remove XForceScreensaver at all:
The first call of XForceScreensaver did help to survive a failed blanking with my buggy intel driver (screenShield vanished, but gnome-shell was able to recover while without it, it was not).

I still don't understand this. Why should the X ScreenSaver extension matter at all?
gnome-shell doesn't use it (we use X Composite to get a window above everything else), gnome-settings-daemon doesn't use it (it uses the DPMS extension to turn off the screen). (Note: gnome-settings-daemon in 3.4 has some bugs related to screen blanking timeouts, but they're fixed in 3.5.90)
Additionally, here my patch works just fine, and if you're experiencing problems, my bet would be again on driver issues.

If it is working for you, then it's indeed very likely that my problems are related to the graphics driver (current intel driver, kernel 3.5, xorg server 1.12.4). Any hints on how debug this properly are appreciated.

I'm ok with it, if you want to commit the patch without addressing my concerns as currently I am the only user who has the mentioned troubles and from a theoretical point of view you are right - no idea why theory and reality in my case are conflicting.

Concerning g-s-d: I've always used at least the most current released version of gnome libraries and apps for my tests, sometimes git snapshots, too, so my g-s-d does no longer have the mentioned issues.

Same problem here. 

Intel driver 2.20.8 (sna)
Kernel 3.5.4
xorg-server 1.13.0 (tried with 1.12 as well)
Mesa 9.0 from git (tried mesa 8.{0,1} as well)

The xset tricks did nothing, haven't tried the patch yet as it didn't work for reporter either.

Currently I'm using the patch modified with the comments I've added and the intel driver with uxa instead of sna as sna still keeps crashing. This combination seems to work more or less.

I've switched to uxa and have no problems so far, so if it's just sna this should probably go to the intel devs.

I've opened a bug at fdo : https://bugs.freedesktop.org/show_bug.cgi?id=55395

This has been resolved upstream, fixed in xf86-video-intel-2.20.9 . See fdo bug as per comment #21 .

The BadDrawable stuff, yes, but not the suspension of xserver's built-in screensaver.

With the recent commit to gsd (Bug 686339), this one seems to be resolved.