Since upgrading Virtualbox from v4.3.22 to v4.3.24, mouse interaction with the guest has gone a bit strange. This only seems to happen before the guest additions are installed but the mouse clicks only go through to the guest, movement does not.
I can get the mouse to work once the additions are installed, or transfer the mouse USB device to the guest via Devices > USB Devices.
Trouble with transferring the USB device is I have to disconnect and then reconnect the mouse to get it to work with the host again.
#13935 (Mouse capture failing upon guest install after upgrade to v4.3.24) – Oracle VM VirtualBox:
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