SUMMARY

There are a number of issues with DisplayPort MST technology with KDE. The main issue that it is introducing severe usability bugs when display is waking from sleep.

STEPS TO REPRODUCE
1. Plug in first monitor to PC using DP input
2. Plug in second monitor to first monitor using its DP output while MST enabled
3. Monitors are working fine. Visible in "Display configuration", movable, you can use them as your ordinary monitors
4. Put monitors to sleep (using timeout in settings or `xset`)
5. Wait for monitors to successfully enter standby mode
6. Awake monitors (keyboard input, etc)

OBSERVED RESULT
1. Monitors are woke up. You may see desktop state from before suspending.
2. Monitors are flickering
3. Their relative position are reset to "right to/top" (I set second monitor to stick right to first but not to the top but to the bottom)
4. Their numbering are changed (e.g. from DP-6/7 to DP-7/8)
5. Global menu (that was present on first monitor) are gone
6. Walpaper on second screen are set to default one
7. Icons on "Activity Pager" widget (panel on first monitor) are wrong (it was set to "show only this screen" and it seems that after awake it shows icon from second screen

EXPECTED RESULT
1. Not having glitches/flickering
2. Not having broken global menu/wallpaper/activities/position

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.69.0
Qt Version: 5.14.2
Kernel Version: 5.4.34-1-lts
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7 CPU @ 3.10GHz
Memory: 32 GB of RAM


ADDITIONAL INFORMATION
Monitors:
1. BenQ PD2700U (DP)
2. NEC E223W (DP through MST)