In addition to the new UI customizer, update 1.2 brought along a couple other display options that are now available. One of the most welcome additions is the “movable secondary windows” option. With this option enabled, you can have multiple windows open at the same time, such as a GTN window, your inventory, and your crew skills window!
Without this option enabled, you can only have one window open at a time. So for example, if you had the GTN window open and you opened your inventory window, the GTN window would close. To enable movable secondary windows, open your UI customization window and look for the option.