Minecraft – I can’t click to pick up & place items in inventory on Windows 8 Minecraft. Can this be enabled

I had the same problem a few days ago.

Go into settings, then video settings and there should be an option that says like "start as maximized" (sorry I'm not at my computer right now so I don't know the exact wording). By selecting that option it should toggle to windowed mode. Now if you restart Minecraft you can change the window size.

You can also press F11 to go back into full screen mode but unless you started minecraft in windowed mode you won't be able to change the size if you go back to windowed mode.

As you already mentioned yourself, this is most likely a ghosting problem. Not quite ghosting, but rather jamming. But these are both rollover problems

"Gaming" mice often register themselves as additional keyboard device to your operating system and they can suffer from the same problems. Either your hardware can't handle the combination of certain keys, or the driver on your computer is not fit to recegnize given combination. (some source here)

It seems like your mouse is of medium quality and does not provide proper key rollover for the thumb buttons. I dont't think there is much you can do besides ensuring that your driver is up to date.