Hit Ctrl-Shift-Z on any website.

AR: Nothing
ER: Should open Debugger

Once Debugger opened via menu, closing and re-opening the Debugger via shortcut works – some part of the UI will be focused then that makes it work. But when the website or browser UI is in focus, the shortcut does nothing

In my case, even with the debugger open, the shortcut keys don't do anything.

Incidentally, executing the debugger statement in an ECMAScript script doesn't open the debugger, either. When the debugger is already open, it doesn't pause. So maybe this bug relates to the debugger's interface, rather than something in the normal browser code.