Developer Tools Console opens by itself randomly. How do I stop/disable this?

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The Problem:

Suddenly and without warning, each time I opened a new tab in my Chrome browser (in Linux, but presumably the same problem exists in all OS's), the Developer Tools Console (window, sidebar) opened automatically along the right side of the tab.

What Changed?

This is the first question anyone should ask after something unexpected happens on your computer. In my case, I had unplugged my keyboard and plugged it back in, in an attempt to correct another unrelated problem. 

Searching the Forums

Not a complete waste of time, because I did actually arrive at the solution below, but what a mess and a jungle of tech talk, developer babble, guesswork and "ifs, ands, ors and buts" the world of forums is.

The Solution

Hit the F12 key once, then open several new tabs to confirm that the fix worked. It worked for me. (Make sure the F12 key is not stuck in the "down" position: mine was!)