Workaround: Change virtual machine setting Pointing Device from "PS/2 Mouse" (the old default) to "USB Tablet" (the new default). Source: https://www.virtualbox.org/ticket/13108. From https://www.virtualbox.org/manual/ch03.html:
Known that for a while, and enabled it for all my virtual machines without Guest Additions available.
"USB Tablet" has apparently been the default pointing device for a new virtual machine in recent versions of VirtualBox. "PS/2 Mouse" was the default for new virtual machines in older versions of VirtualBox. I changed it to "USB Tablet" in most of my existing virtual machines. Even if a given virtual machine has Guest Additions installed, "USB Tablet" is useful if you're using a boot cd/dvd/usb in the virtual machine.