Bug 1570311

Created attachment 1425110 [details]
This is the verbose log that was generated when I was testing the reproducable steps and part of the original email.

I have emailed Jamie Zawinski about this issue and he doesn't know why this would go wrong or how this could break in this way. The following was in the email that I sent him: 

I have found that after changing the Compositor settings in Window Manager Tweaks in Xfce4 and locking the screen, there is blank and there is no black background (I have no hacks set to run in the background) and instead a transparent background which allows me to see my desktop before logging in.

How to reproduce:
1. Have Xfce4 4.12 on Fedora 27 (this is all I have tested this on)
2. Open up Window Manager Tweaks and go to the Compositor tab.
3. Uncheck everything under enable display compositing but not display compositing itself and slide all the sliders to Opaque
4. Force a lock with xscreensaver-command -lock
5. The screen has not blanked out and is transparent and continues to do so when giving input for the password popup.

If the XScreenSaver had just been exited and executed again, there is a chance that the screen will blank. But, if "Display fullscreen overlay windows directly" without quotes in Window Manager Tweaks is checked and unchecked again, then the aforementioned issue occurs again.

The work around to this is to just keep "Display fullscreen overlay windows directly" without quotes checked.  

I am using Fedora 27 updated to the latest packages at the time of writing, XScreensaver 5.38 (binary installation), and Xfce4 4.12 with Xfwm with the following --version output:
    This is xfwm4 version 4.12.4 (revision 7844952) for Xfce 4.12
    Released under the terms of the GNU General Public License.
    Compiled against GTK+-2.24.31, using GTK+-2.24.32.

    Build configuration and supported features:
    - Startup notification support:                 Yes
    - XSync support:                                Yes
    - Render support:                               Yes
    - Xrandr support:                               Yes
    - Embedded compositor:                          Yes
    - KDE systray proxy (deprecated):               No

In case the X server matters this is the output of Xorg --version:
X.Org X Server 1.19.6
Release Date: 2017-12-20
X Protocol Version 11, Revision 0
Build Operating System:  4.15.14-300.fc27.x86_64
Current Operating System: Linux yetoo-Inspiron-570 4.15.15-300.fc27.x86_64 #1 SMP Mon Apr 2 23:14:02 UTC 2018 x86_64
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-4.15.15-300.fc27.x86_64 root=UUID=6703edab-3760-44f1-a266-2c74a47d23c7 ro LANG=en_US.UTF-8
Build Date: 12 April 2018  03:55:14PM
Build ID: xorg-x11-server 1.19.6-7.fc27
Current version of pixman: 0.34.0
    Before reporting problems, check http://wiki.x.org
    to make sure that you have the latest version.