(Ctrl-Tab. Cmd-Tab is something else.)

There are various shortcuts used in different browsers and apps, and we'll support most of them. Tab switching is partly implemented already with Ctrl-PgUp/PgDn (which is Ctrl-Fn-Up/Down on a Mac keyboard), but focus currently gets lost easily. That will be fixed, and the additional shortcuts will be supported.

I'd like to second this request, or to request an improvement to this shortcut.
On my Mac, the keyboard shortcut ctrl-PgUp/PgDn functions as described ONLY when the currently active tab is my library and when I have selected a field with the cursor. Any field can be active, so long as I have clicked on either a library folder, tag, item, or a field in the item pane.
However, the shortcut does not work if the currently active tab is a particular item, no matter what field is active.
Is this perhaps because the pdf viewer is still in beta? Note that the shortcut doesn't work even if I highlight a field in the item pane. What seems to matter is that the tab is not the main library tab. In this situation, the only way I can find to navigate back to the library tab is with the cursor.

We've added support for additional shortcut keys for switching tabs in the latest beta (and fixed the problem where they didn't always trigger based on the current focus).