External Displays Turn Off After Only A Few Seconds Of Inactivity

OS VERSION
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=17.04
DISTRIB_CODENAME=zesty
DISTRIB_DESCRIPTION="Ubuntu 17.04"

UPOWER VERSION
upower:
  Installed: 0.99.4-4
  Candidate: 0.99.4-4
  Version table:
 *** 0.99.4-4 500
        500 http://us.archive.ubuntu.com/ubuntu zesty/main amd64 Packages
        100 /var/lib/dpkg/status

EXPECTED BEHAVIOR
Expected displays to stay on while idle, for at least a few minutes (per Power Settings).

WHAT HAPPENED INSTEAD (and hardware information)
I just upgraded from a fully-updated Ubuntu 16.10 64-bit Desktop installation to 17.04, using the update manager. In 16.10 and 16.04 LTS, this issue did not exist for me.

I'm using a MacBook Pro with Ubuntu installed in a dual-boot configuration. I have two attached monitors: one via the mini DisplayPort and one via HDMI. I normally keep the laptop lid closed while working. Since the upgrade (persisting through several reboots), whenever I have the notebook lid closed, if I don't touch the keyboard or mouse for about 5 seconds, both external displays go into power-saving mode. When I touch the keyboard/mouse, again, they instantly come back up (no delay), indicating that the computer had probably not been suspended. If I then don't touch the keyboard/mouse for another 5 seconds, they immediately go back into power saving mode. (Repeat...)

Going into Display Settings and changing "Built-in Display" to on or off doesn't seem to have any effect. When I close the lid, it changes itself back to "OFF"; and when I open the lid, it changes itself back to "ON". (This seems like a sensible behavior.) Notably, the on/off switch doesn't seem to move unless I exit and re-enter Display Settings.

I found a workaround to this problem on a forum post:
In "/etc/UPower/UPower.conf", change "IgnoreLid=false" to "IgnoreLid=true".

After doing this, the Built-in Display switch stays on whatever I set it to, regardless of whether the laptop lid is open or closed (which probably isn't ideal); but more importantly, the external displays no longer turn off when idle for a few seconds. They now obey the settings I explicitly define in Power Settings.