Cycle through ALL open windows in ALL open applications? (keyboard shortcut?)

We all know Cmd ⌘ Tab ⇥ to switch apps, and Cmd ⌘ ' to cycle though windows within an app, but…

… How to cycle through ALL open windows in ALL open applications? (keyboard shortcut?)

I knew this once, but forgot it. What is the key combo for this?

There's Ctrl ⌃ F4 which is "Move focus to active or next window"… but it doesn't work across Spaces properly.

You could use Cmd ⌘Tab to switch apps, and on the icon of the application you want to switch windows in press the down arrow key (with Cmd ⌘ still pressed). Then use left/right keys to navigate to the desired window across spaces and desktops.