GNOME won't let you watch your videos in peace
Screensaver control on the Linux desktop is a total mess, but
the xdg-screensaver command provided an interim
cross-desktop solution by detecting and using the appropriate
underlying interfaces. For GNOME, it depends on
the gnome-screensaver-command --poke option, but that has
been broken and then
removed in favour of the cross-desktop D-Bus API
org.freedesktop.ScreenSaver
another
GNOME-specific API which can't practically be used by
xdg-screensaver.
What's more, switching to the new API seems to have introduced various bugs in GNOME's own video player, Totem, such that it often failed to inhibit the screensaver in GNOME 2.32 and 3.0. Apparently everything is wonderful again in 3.2; here's hoping.
(In case anyone asks where my bug report on the totem package is, I already looked at the bug list and it's clear that there's no point.)
Updated: Well I said the GNOME Session D-Bus API couldn't practically be used in xdg-screensaver, but then set out to prove myself wrong. And with the aid of Perl, I may have succeeded.