That happens when the download list goes into autohide mode. Click view and select download list. This will get the download list displayed and the tabs on the bottom. Next if the little push pin if pointing down click on it and make sure it is pointing to the side. This should take care of your problem. Happens to me sometimes when I switch windows to say my Thunerbird mail.
I was going to report this to you guys, but I have been busy today. The honey do list is up and running
Aaron
Sorry got that backwards the push pin needs to pointing down to keep the down load list from autohiding.