Host : Debian Guest os : Win10 In full screen mode, it cannot use CTRL+mouse for multiple selection

[Edison]

Host OS: Debian 10
Guest OS : Window 10 20H2 \ Window Server 2019
VisualBox + Extension Pack : 6.1.22 r144080

In full screen mode, it cannot use CTRL+ mouse for multiple selection ;
In other screen mode , it's normal to use CTRL+ mouse for multiple selection ;
In host system , it's normal.

[mpack]

You are describing a guest app feature, i.e. how it interprets a certain combination of inputs. I suggest you look to the guest app to explain why it doesn't do as you expect.

The only time this could be laid at the VirtualBox door is if VBox somehow was not passing along mouse clicks combined with keyboard presses to the VM, and I'm quite sure that is not the case. I don't see how the screen mode can be relevant to mouse or keyboard events.