Inconsistent and erratic mouse wheel in Linux while moving the mouse pointer

I found this post on the VirtualBox forums in a thread that exactly describes my problem. It seems to be unrelated to VirtualBox, as VMWare shows the same behavior.

The solution that worked for me:

1. Check that you're currently using libinput.

   $ grep "Using input" /var/log/Xorg.0.log
   [     0.000] (II) Using input driver 'libinput' for 'Power Button'
   [     0.001] (II) Using input driver 'libinput' for 'Sleep Button'
   [     0.002] (II) Using input driver 'libinput' for 'Video Bus'
   [     0.003] (II) Using input driver 'libinput' for 'VirtualBox mouse integration'
   [     0.004] (II) Using input driver 'libinput' for 'VirtualBox USB Tablet'
   [     0.005] (II) Using input driver 'libinput' for 'AT Translated Set 2 keyboard'
   [     0.006] (II) Using input driver 'libinput' for 'ImExPS/2 Generic Explorer Mouse'
   [     0.007] (II) Using input driver 'libinput' for 'VirtualBox USB Tablet'
   

2. Install the evdev input drivers.
   On Manjaro this is found in the xf86-input-evdev package, which was already installed.

3. Enable the evdev drivers by modifying the X11 configuration.
   In the directory /usr/share/X11/xorg.conf.d/ I already had a file 10-evdev.conf with the evdev configuration. It was just overridden by the higher-priority 40-libinput.conf configuration. So all I had to do was:

   cd /usr/share/X11/xorg.conf.d/
   sudo mv 10-evdev.conf 80-evdev.conf
   

4. Restart.

5. Verify that the evdev drivers are now used instead:

   $ grep "Using input" /var/log/Xorg.0.log
   [     0.000] (II) Using input driver 'evdev' for 'Power Button'
   [     0.001] (II) Using input driver 'evdev' for 'Sleep Button'
   [     0.002] (II) Using input driver 'evdev' for 'Video Bus'
   [     0.003] (II) Using input driver 'evdev' for 'VirtualBox mouse integration'
   [     0.004] (II) Using input driver 'evdev' for 'VirtualBox USB Tablet'
   [     0.005] (II) Using input driver 'evdev' for 'AT Translated Set 2 keyboard'
   [     0.006] (II) Using input driver 'evdev' for 'ImExPS/2 Generic Explorer Mouse'
   [     0.007] (II) Using input driver 'evdev' for 'VirtualBox USB Tablet'
   

   This however didn't yet fix my problem. Apparently I needed imwheel too.

6. Install imwheel. On Arch, I had to install the imwheel AUR package.

7. Run imwheel to verify that this fixes the issue.

   $ imwheel
   

8. All that remained was to make imwheel run at startup. I run this command, as it only intercepts the scroll wheel:

   imwheel -b 45
   

This fixed the issues! Scrolling is now working correctly in all applications.