Virtualbox: 2-Finger-Touchpad-Scrolling on Win10 Guest very choppy (Ubuntu 20.04 Host)

After finally taking the plunge and upgrading from 18.04 to 20.04 I think I've hit the first snag:

• 2-Finger-Touchpad-Scrolling (Thinkpad X1) on Win10 Guest has become very choppy / unusable (Ubuntu 20.04 Host).
• In contrast to the touchpad, scrolling with a USB mouse wheel works well/normally/smoothly.
• Guest Additions on the Win10 Guest are installed.

Google so far yielded two relevant hits:

1. https://forums.virtualbox.org/viewtopic.php?f=7&t=93857&start=15 and
2. https://ubuntuforums.org/showthread.php?t=2444679.

The solution propounded by the former (running QT_XCB_NO_XI2=1 VirtualBox in terminal) unfortunately did not do the trick yet for me... The latter does not yet offer a (bug?) fix / solution, but only confirmation of the problem, as it were.

How to fix?

This is the proposed solution from number 1. above (which doesn't seem to work for me):

The problem is that XInput2 smooth scroll events are not being received by the guest.

To see this, listen for XInput2 events on the host and the guest by

xinput --test-xi2

When scrolling, only the ButtonPress and ButtonRelease events generated by the host will be received by the guest. The smooth scroll Motion events are never received. This is why scrolling vigorously allows some scrolling to work, since this is the only time that ButtonPress and ButtonRelease events are generated by the host.

To work around this issue, you can run VirtualBox with the QT_XCB_NO_XI2=1 environment variable set. This disables QT's usage of XInput2 and has it fall back to using vanilla XInput scrolling behavior. In other words, to work around this issue, from a terminal, you can run

QT_XCB_NO_XI2=1 VirtualBox

This works because for vanilla XInput clients, the X server is required to emulate smooth scrolling using only ButtonPress and ButtonRelease events, which are the ones, at present, capable of being received by the VirtualBox guest.