I don't use the Backstage and still use the old legacy Open dialog box to open documents. I have not found a single keyboard command to move the focus and cursor from the File name: input box to the documents list. Like how for example Ctrl+F moves the cursor into the Find input box.

I am aware Shift+Tab TWICE does it, but it feels like an obvious shortcut that should exist is missing, one I would use several times a day. Oddly, Shift+Tab TWICE doesn't even select the first document in the list, it places a box around it and I have to down arrow and up arrow if I want to select the first document in the list.