This could be caused by an application inhibiting the screen saver. An example is chromium. Generate trace with the following command:
Here is the output I got when running chromium:
TRACE[xfpm-inhibit.c:405] xfpm_inhibit_inhibit(): Inhibit send application name=/usr/lib/chromium-browser/chromium-browser reason=WebRTC has active PeerConnections sender=:1.223 TRACE[xfpm-inhibit.c:99] xfpm_inhibit_has_inhibit_changed(): Inhibit added TRACE[xfpm-power.c:959] xfpm_power_inhibit_changed_cb(): is_inhibit TRUE, screensaver_inhibited FALSE, presentation_mode FALSE TRACE[xfpm-power.c:984] xfpm_power_inhibit_changed_cb(): is_inhibit TRUE, screensaver_inhibited TRUE, presentation_mode FALSE
The page that caused this situation is:
https://codereview.stackexchange.com/questions/205553/c-math-vector-template
Chromium showed me that a webrtc was active by going to this page in chromium:
'chrome://webrtc-internals/'
With this page running and the inhibit signal being sent the screen will never blank. Closing the tab fixes the problem as can be seen by the following debug output:
TRACE[xfpm-inhibit.c:423] xfpm_inhibit_un_inhibit(): UnHibit message received TRACE[xfpm-inhibit.c:93] xfpm_inhibit_has_inhibit_changed(): Inhibit removed TRACE[xfpm-power.c:959] xfpm_power_inhibit_changed_cb(): is_inhibit FALSE, screensaver_inhibited TRUE, presentation_mode FALSE TRACE[xfpm-power.c:984] xfpm_power_inhibit_changed_cb(): is_inhibit FALSE, screensaver_inhibited FALSE, presentation_mode FALSE ……………
While evaluating this problem I learned about the xset dpms force off command which blanks screen immediately. However, with the tab still open in chromium the screen won't stay off for long following the issuance of this command.
Useful commands include: