Keyboard shortcut to select the full directory path of the active window

Alternatively, you can use Shift+Context Menu, A (note that this may differ if you have non-standard shell context menu entries with overlapping mnemonic/access keys). This may produce more keystrokes on it's own, but you can circumvent that with an .ahk script to achieve a single-input solution. You can technically bind any of the mentioned methods through AHK actually, but it can be a bit of a learning curve for those not aquainted with the scripting nuances, particularly when multiple modifier keys are involved.

In Windows Explorer and most web browsers, hit Alt-D to select the path in the address bar then Ctrl-C to copy.

This Alt-D keyboard accelerator recalls the URL bar in Internet Explorer 6 and earlier, which was preceded by the word "Address" with an underlined "d":

If you are using File Explorer (Windows Explorer), you need to combine different keys. Press F4 then Ctrl+A followed by Ctrl+C

Or

As mentioned by @hextech on the comments, use Ctrl+L, then Ctrl+C