Turned JavaScript back on and the hijack reappeared.
All for now, gotta hit the sack. Thanks!
On my iPad, the hijack issue is still there whenever JavaScript is turned on. I've reset the iPad now numerous times, so it obviously survives a restart. It's quite persistent, and makes view the Dilbert blog posts essentially impossible. So I'm turning JavaScript off to view Adams' posts, which is a minor PITA, but it makes me wonder...
Meanwhile, in Settings -> Safari -> Advanced -> Website Data there is is an unremovable entry for "dilbert.com" that won't go away when I delete the website data (either eith Remove All or using Edit and deleting it individually. That's troubling.
And on my Mac desktop (El Capitan), I don't see the hijack, but in the exact same place, I see two similar overlays allegedly for "Tumblr" -- one that (according to the mouse-over status line) takes me to the "scottadamsblog" area of Tumblr, and the other takes me to Tumbler with a reference to the blog page.
It seems oddly coincidental that the two overlays are the same color scheme and in the same position as the "Open" and "Install App" of the hijack on the iPad.
So at the moment my iPad, which has been an essential part of my daily relaxation time, has come under suspicion. Damn.
I'm going to write to Scott Adams' website folks and see if they know anything.