Last image shown before welcome screen shown on resume

So, looking at this, we could just skip the animation and trigger a show() on the greeter when the power comes back on. But there's still a race. So the best solution would be to show the greeter when the screen turns off.

But how do we know when that happens? DPMS doesn't seem to have signals (and that's an X thing anyway). Mir doesn't yet seem to have much of an API to expose that info, and I'm not sure it plans to.

GNOME exposes it over DBus via settings daemon, but we're not running that in Touch.

I guess we can monitor /sys/class/backlight/acpi_video0/actual_brightness but that seems really low level? Qt doesn't seem to abstract this at all for us. I'll dig a bit deeper, but for now, monitoring /sys is my best avenue.

Aha, this API can be used: http://doc.qt.digia.com/qtmobility/qsystemdisplayinfo.html

That, plus skipping any animation on show() should fix this. (We still need to skip the animation since there is a race even just turning screen off and on very quickly.)

I'm marking this Fix Released. The "show greeter when screen goes black, rather than when it turns on" bit was done a while ago.

The remaining "skip show animation to avoid a race condition with turning screen off/on quickly" bit doesn't seem possible. The user can turn screen off/on faster than unity8 gets and handles the signal from powerd. So even if we drop the animation, there's still a race where the user can see their dash for a split second before the greeter swipes in. Since that race still exists, it's more pleasant to at least show the animation for the greeter if the user does find themselves in that situation.

Alternatively, don't lock the screen if the user switches it back on that quickly. Pretty sure Android does this, so the phone doesn't lock until something like 10 seconds after flicking it off.