Version 15.1.1.

Using CTRL + [ when the CC Libraries Window/Tab is open closes the window on the workspace instead of sending the selected object backwards. I have to manually go back to the Toolbar and check CC Libraries so it can reappear again only for it to close out again when I use CTRL + [ unless I manually click on my artboard or close out of the window first before using the shortcut. Maybe it's a shortcut that I'm unaware of, and if it is I think it's being shared with a shortcut that is used too frequently for it to be an efficient shortcut. I have checked to see if the keyboard shortcut closes other windows (I tried it on just about every default window) but the CC Libraries window was the only one that it kept closing.