I'm not sure if this is a VirtualBox or a KDE problem- hopefully, somebody will know more about it.
I'm using VirtualBox 5.0.12 with Kubuntu 14.04 both host & guest (my reasons), and updated guest additions. I have configured the host KDE to lock the screen on lid close (with the standard System Settings / Power Management / Button Events Handling). This works fine in general, with one exception- if the guest is running in full screen mode, the host does not lock the screen. In this case, closing the lid does nothing, no messages, nothing relevant in /var/log/syslog or Logs/VBox.log. The guest has no reactions either, though I don't care for it to react, I want to lock the host, not the guest. When the guest is in windowed mode, even with focus, closing the lid locks the host as expected.
By using acpi_listen on the host, I can confirm that the host OS continues to receive the lid close events, even with the guest in full screen. However, the host KDE does not seem to react to these events. Is there anything I can do to get the host KDE to lock the host in such cases?