Config:  HP envy laptop, Intel i7 quad core, Intel graphics, Fedora 23 dual boot with Win 10.

Problem:  External display not detected on hdmi port.

When I either plug in a new screen into the hdmi port (this box doesn't have a vga port) after booting *or* power down, plug in the hdmi cable and then power up, the external display is not detected.  There are no messages in /var/log/messages (I'm not sure where else to look) about it, and the second display does not show up in  System Settings.  

The second display doesn't seem to notice the computer, either -- normally the display moves from power-save mode to "on" when the computer comes up, but it doesn't detect the box.  Instead, I get the message from the display "In power save mode.  Press any key on keyboard or move mouse."

The second display is recognized and can be configured in either Win 10 *or* the live (usb) version of Linux Mint 17.3 (KDE 4.3).

Procedure:

1. Turn on box and boot up KDE 5.5
2. Plug in hdmi
OR
1. Plug in hdmi
2. Power up box and boot up KDE 5.5

3. Open System Settings -> Display and Monitor
4.  Shout obscenities because you only see the internal display.