The way I've found to do that is to click on your tech bar at the top of the screen, which will deselect the current tech. Then right-click on an available tech to bring up the civilopedia. While that is up you can press F1 to bring up a list of your cities, then you can click into your cities to look around if needed. Or use some of the other F-keys to look at whatever you need (F4 for diplomacy and trades comes to mind). Also, you can click on the mini-map while the trade window is open and drag around down there to move the main display behind the trade window. As long as you don't hit escape you can still take action on the active trade offer.