I watched a patron use three keys to open a child in a new tab from the parent's screen, but I couldn't see which keys he used.
for windows 10, I would just hold down the <control> key and click on the name of the child at the same time to open a new tab from the parent's screen.
for a Mac machine, I would just hold down the <apple> key and click on the name of the child for the same results.
Was the patron using their own device?
Many web browsers have keyboard shortcuts, but they are not all the same. There are keyboard shortcuts also in the operating system, in the Accessibility preferences tool. And there are add-on software tools that modify keyboard functions further.
Next time you see the patron you might ask what they are using.
On my system a mouse right click opens links, but I often prefer to use the keyboard.