So I ran into a weird issue today with my second monitor. Upgraded to April 27th -current last night and no issues. Today, my second monitor will only run at a max resolution of 1024x768. This is a 17" monitor I run in portrait mode, normally at 1280x1024, rotated. It also no longer seems to detect any details from the monitor, such as make and model. Just that it is attached to DP-1. When I dual-boot into Windows 10, it runs at full resolution, and displays the monitor details. Windows 10 did act like this was the first time I attached this monitor though.
I am running slackware64-current, xfce, and nvidia driver 352.79 on a GTX 950. I was running nvidia driver 362.41 successfully, but downgraded in an attempt to solve this.
The only thing I can think of that would do this is the upgrade to xfce-settings yesterday, but this seems more xorg or kernel driver related. Same issue when I log into KDE.

Have you tried re-installing the 362.41 nVidia driver and running 'nvidia-settings'?

I just did. nvidia-settings will not let me change the resolution above 1024x768. I did remove the nvidia driver, reinstall xorg-server and mesa, then reinstall the driver through the .run package earlier today.
Part of me says it's a recent update from current, because that's all that has changed, and it works in Windows still. Part of me thinks it's hardware because of it seemingly not able to read the EDID from the monitor correctly. It almost looks like its defaulting to a safe state.
Going to try liveslack.

EDIT: Well, second monitor used to work in liveslack. Not working at all now. nouveau driver did report an EDID error from the second monitor. xrandr --props does not report an EDID at all from the monitor, but it does for the primary monitor.
I will start looking at this like a hardware issue.

On closer investigation this does indeed appear to be a hardware issue. Windows 10 is reporting it as a non-PNP monitor, but is able to run it at its higher resolution. The timing after doing an upgrade to xfce4-settings was just coincidence.

The nVidia driver accepts a 'UseEDID' option that could be used to ignore the bad/non-existent EDID information.
The 'cvt' utility can be used to generate an appropriate modeline for the monitor, based on the monitor documentation.
The generated modeline can be tested using the 'xrandr' '--newmode' and '--addmode' options.
With appropriate entries in /etc/X11/xorg.conf you should be able to get the monitor to work as before.

PS - If your second monitor is the same as your first, then you could capture the EDID from the first monitor and associate it with your second monitor using the 'CustomEDID' option to the nVidia driver.