After doing an update (Arch) on 2018-11-27 (including mesa (18.2.4-1 -> 18.2.5-1), linux (4.19.2.arch1-1 -> 4.19.4.arch1-1)) I found I had corruption on the center monitor of a PLP setup. Further investigation found that the corruption does not occur if the large panel is configured to be on the left (both using XFCE display settings, and the equivalent in KDE). This was replicated on both my installed OS and a Manjaro 18.0 live USB.

Corruption takes the form of every other line being offset by an increasing amount, although it only starts part-way down the monitor. Side monitors are not affected.

https://i.imgur.com/0pQMznF.jpg

Probably important details:
- Arch / XFCE / XDE / lightdm
- AMDGPU open source drivers (Sapphire Radeon RX580)
- 3 monitors in PLP: sidecars are 1680x1050 (but turned so they're vertical), center is 2560x1600. Left-hand sidecar is on HDMI, other two are DP.
- Corruption DOESN'T happen if the large monitor is placed on the left! Nor does it happen at the login screen (which doesn't have any screen rotation and probably also has the big panel on the left)
- Corruption DOESN'T happen if I reduce the resolution of the center screen to 2048x1280 (but obviously that's not a solution)
- Corruption DOESN'T happen if I pass amdgpu.dc=0 to the kernel
- Corruption DOES happen on KDE (tested with Manjaro KDE live USB) as long as you arrange the monitors in the same way

Here's a paste of the pacman.log from the day it all went wrong:
https://pastebin.com/H3i5LuzU
or
https://hastebin.com/dixemisepo.php

Thank you for your attention.