DisplayPort monitor suddenly started not using the full screen, under-scanning for less than highest resolution

I have two HP Z27s 4k monitors connected via DisplayPort to my hp PC running Windows 7. Due to issues related to font scaling, I've been running both for at least a month at 2560x1440 without significant problems. [I was getting some random banding that powering the monitor off and on fixed.]

Today, one of the monitors started flickering ~2"-tall horizontal bands of noise at random places on the screen. In an effort to fix that, I set the Refresh rate to 30 Hz (it was 59 Hz on both monitors). When I did that, the display on that monitor shrunk down to not being scaled at all. That is, there appears to be a 2560x1440 display centered on a 3840x2160 monitor, with significant (4" R & L, 2" T & B) underscan, equivalent to the pixel difference.

I of course immediately selected "Revert", but no joy. Now, no matter what I set the Resolution to on that monitor, if it's less than 3840x2160, I get the same effect: the selected # of pixels centered in a fixed 3840x2160 field. [And if I set it to 3840x2160, the monitor keeps going black for a second or two every several seconds.] If I swap the monitors' ports, the same monitor behaves that way (but stays my left-arranged monitor, which leads me to believe that Windows was able to identify them uniquely, so it might still be something in Windows).

Oddly, when Windows reboots, the animated Windows Starting up screen does take up the whole display, but then when the Windows signon screen appears, it's back to 2/3 of the full display.

What did I screw up? I've looked everywhere. Did I kill a dying monitor? Have I cooked the integrated graphics adapter?

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