I tried Alt-Click, Ctrl-Click, and Shift-Click. One of them puts the file in the Other Panel, one adds a second chapter to it, and the last puts all the chapters from the first chosen file to the last one in the Other Panel.
But I want to open up a file in the Quick Reference panel! I tried to add Alt-LeftClick or Alt-RightClick into the keyboard shortcut for it, but it didn’t work.
There doesn’t seem to be a way to change the options to make double-click do it either.