Feature request: shortcut to focus previously focused window

[syl20bnr]

Is your feature request related to a problem? Please describe.

We have now a shortcut to focus the main window directly with Shift+Cmd m. It is awesome but we cannot directly focus back the previous window.

This UX issue is especially annoying in Two Panes layout with more that 2 windows.

Describe the solution you'd like

We need a shortcut to go back directly to the previous focused windows so both actions are O(1).

Describe alternatives you've considered

The alternatives is to cycle through an arbitrary number of windows to go back from the main window to the previously focused window using Shift+Cmd j or Shift+Cmd k.

Additional context

N/A

[wtianyi]

Mostly this can be done with the system window-switching shortcut of macOS: ⌘-<Tab>, except that this shortcut will work among windows across screens, so in some scenarios it will not do the job. For example, assume one is working on the secondary pane of Two Pane layout in screen 1, and the following actions are performed:

1. Focus the main pane on screen 1
2. Focus the screen 2
3. Focus back to screen 1

Now, ⌘-<Tab> will not focus the secondary pane, but the last active window in screen 2, and one may have to use many key strokes or the mouse to focus the secondary pane on screen 1.