By default, the mouse is captured if the user clicks on the guest window and the guest expects relative mouse coordinates at this time. This happens if the pointing device is configured as PS/2 mouse and the guest has not yet started the Guest Additions. For instance, the guest is booting or the Guest Additions are not installed, or if the pointing device is configured as a USB tablet but the guest has no USB driver loaded yet. Once the Guest Additions become active or the USB guest driver is started, the mouse capture is automatically released.

The default behavior is sometimes not appropriate. Therefore it can be configured as follows:

VBoxManage setextradata VM-name GUI/MouseCapturePolicy mode
                  

mode is one of the following:

Default

The default behavior as described above.

HostComboOnly

The mouse is only captured if the Host Key is toggled.

Disabled

The mouse is never captured, also not by toggling the Host Key

This is a per-VM setting.