Yet another marvellous new 'feature' – Safari 11.1 now will not auto-fill any login data unless you specifically click the first field, select the login drop menu, select the login….
Is there any way to go back to the old method & prevent this complete & utter waste of time, every time?
To clarify, after comments.
The mac is unlocked, I'm sitting at it working. Security is not at issue; if I leave my machine, either I will lock it or it will self-lock.
Any web site I visit that requires a log/pass [already saved in Keychain, which is also unlocked] then requires me to click in the first box. Safari then presents a credentials drop menu for that box. I then have to click on it to populate the fields.
This does not improve security, it merely wastes time.
This is in no way an improvement on simply auto-filling with the same information, which it used to do until recently.
I could understand it if there were potentially multiple entries to choose from; but that would be an edge case not a standard practise.
Whether or not there are 'hidden text fields' is not really an issue, as this data was previously manually entered into the visible fields then saved to Keychain. Keychain wouldn't populate fields with data it had not already saved for that page.
This is not for the the 'personal info autofill' – 
– which already needs specific user interaction for good reason, only site-specific previously saved Keychain log/pass information.
Best Answer
This is only a partial answer covering a slight alleviation for:
My personal grievance with this situation is that there seems no way to use the keyboard for this and being forced to use the mouse/trackpad to hit a usually tiny caret to achieve the desired result. Using the keyboard for this task would be at least much faster and less error prone.
Unfortunately, it looks as if you still may use Tab to get to the username field, but after that all keyboard actions seem to be inert and it's back to using a pointing device?
That seems to be amplified by a bug. If you Tab past the username field and into the password field, then Shift+Tab back into the username field, then the information is auto-filled in for the most recent credentials and you may even select other identities with the cursor keys.
This behaviour differs depending on the page in question and also has its level of practicality depending on the page design: how many fields there are to tab into, before reaching the desired one.
On a relatively cleanly designed page like Apple's developer program login page the behaviour is as described above. On other pages the behaviour can sometimes be different and even be more like it was before. For example, Microsoft dedicated login pages preselect an open field and it's just a cursor down + Return to go forward.
This was tested on 10.12.6 and with Safari Version 11.1.1 (12605.2.1).