This is an issue of “VBoxGuestAdditions_6.1.4.iso”. To fix it, we need to downgrade it to the previous version “6.1.2” inside the VM. The solution comes from here.
# Run the following commands inside your VM.
Updated on 2020-04-16.
This issue is fixed in the latest VirtualBox “6.1.6“ version. Here is the changelog:
VirtualBox 6.1.6 (released April 14 2020)
This is a maintenance release. The following items were fixed and/or added:
GUI: Multiple enhancements including visual elements updates
Graphics: Fixed monitor resizing and multi-monitor handling bugs on X11 guests with VMSVGA graphics adapter
Graphics: Enhancements in 2D and 3D acceleration and rendering
USB: Multiple enhancements improving prformance and stability
Serial port: Improve error handling and fix hang when host port disappears
VBoxManage: Multiple fixes for guestcontrol operations
API: Fix for exception handling bug in Python bindings
Shared clipboard: Multiple fixes including possible crash and HTML data support
Linux host and guest: Support Linux kernel 5.6 (bug #19312)
In my option, this issue is so low level that I couldn’t help thinking that whether VirtualBox had a basic UI automation test process or not.