Panels will only dock on left and right sides, not top or bottom

Using latest release of inCopy on macOS 10.12.4.

According to the documentation (and as demonstrated in a Lynda.com video), panels can be docked on the top and bottom of the screen in addition to the sides.

However, when I try to do this, I do not get the blue line/highlight that indicates a panel will dock when the mouse is released.

I have restarted the app, the OS, and have trashed preferences, to no avail.