KDE: Parts of system UI are oversized

Some time ago I switched from Windows to Linux Mint and now hopped over to Manjaro.

After booting into my live system and also after installing it to the PC, I saw that most parts of the system UI are completely oversized (see screenshots).
Screenshot_20200104_230722|690x388

The taskbar looks fine, the welcome dialog was also correctly scaled. A wrong size have the system apps like the settings, konsole, dolphin, etc.
Further are the panel decorations way to big, desktop icons are huge and the start menu is nearly unusable.

I use the proprietary nvidia drivers.
All packages are up to date, the screen resolution is set to 1920x1080 and the global scale is at 1.

How can I fix this?

It looks as if you have the standard, out-of-the-box Manjaro theme. Two ideas:

  1. In System Settings, if you type size in the search box, can you adjust your icon size and font sizes?
  2. Have you tried creating a new user?

What is global scale is set to? (system settings > display configuration). (oops: i'm bad at reading)

By any chance are you on plasma-wayland session?
Do you use custom settings for fonts?
Mine are like this:

image

I'm not using wayland and I have the default settings (problem persists even after reinstall).

I looked through different apps and I think it's a problem with the graphics framework:
Fine are:

  • Pamac (Gtk3)
  • Steam
  • Firefox / Thunderbird (except the window titlebar)

Problems have:

  • Dolphin, Terminal, System Settings (all Qt5)
  • Qt Designer (Qt5)
  • VLC
  • and every other system app using Qt5

New users are affected in the same way (the login screen is already).
Font and icon size changes have no effect. Increasing the global scale or reducing the resolution make it even worse.

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