SUMMARY
video area is at unusual place and controls are displayed multiple times (tested with vlc, dragonplayer, smplayer)

multimonitor setup
- laptop screen 1920x1050
- external hdmi connected screen 5760x2160
- fractional scaling

This is not true
 - with a single monitor enabled and fractional scaling enabled
 - with both monitors enabled and fractional scaling disabled
I didn't recognize unusual things in other programs (ex. videos in firefox are working).

VLC
- resized on laptop screen
  - the video is croped at top
  - black space between video and controls
  - controls duplicated 3 times above each other at the bottom of the window (all three contros are functional)
- resized on external screen
  - video tends to be far left (only visible with a big window) or not visible
    - moves left if window height decreases
  - black space right of video
  - controls duplicated up to 3 times (depending on window widths) beside each other at the bottom ob the window (all three contros are functional)
    - only the right part of controls is visible (audio control and right part of video progress slider)
- if a window is draged to the other screen the window keeps its content (applies to all players)
    
smplayer
Same behavior as VLC but controls appear only two times

dragonplayer
- resized on laptop screen
  (description of window content from top to bottom)
  - ~lower third of controls
  - video cropped at top
  - 3x black bar with twice the height of the status bar (seperated by gray line)
  - second status bar
  - black or artifacts containing bar with same height as status bar
  - status bar at the bottom is displayed correctly
- resized on external screen
  - same as VLC


SOFTWARE/OS VERSIONS
Linux: 5.3.0-40 / x86_64
Distro: ubuntu-18.04 / kde-neon User Edition
KDE Plasma Version: 5.18.3
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1

ADDITIONAL INFORMATION
X-Server Release 1.19.6
X-Server Version 11.0
OpenGL 3.0 Mesa 19.2.8

Laptop screen:
display: 4th Gen Core Processor Integrated Graphics Controller
driver: i915
refresh rate: 60Hz

External screen:
display: GK107GLM [Quadro K1100M]
driver: nouveau
refresh rate: 30Hz