MacBook Air not filling entire screen on Dell U2415 External Monitor

I have an early 2015 MacBook Air, and the image is not filing the entire screen on a Dell U2415. I've tried mirroring the displays and selecting "Best for Dell U2415", resetting the NVRAM, rebooting, and running diagnostics on the monitor (all is fine—screen fills to the edge) and every combination of power cycling/sleeping both the MBA and the monitor. There is nothing in the the monitor's settings for underscanning or adjusting the vertical/horizontal size. I'm running it at its native resolution of 1920 x 1200

The same issue was reported here:

https://discussions.apple.com/thread/7945034

Two things about this are strange. In the above post, a user reported that it didn't work for them in whichever Sierra version they were running, but it did work in Yosemite. I'm running Yosemite. The other thing that kind of has my head spinning is that I would swear that it was working fine last night when I first hooked it up because I just bought it and was checking everything closely to ensure it was working. I would have noticed this.

The only difference is that, last night, I had the monitor turned upside down and rotated 180° in the Display Preferences because it clears the arm on my monitor mount more easily when upside-down. I turned it top-side-up because, for some weird reason, the text cursor displays about 1/4" above and to the right of where the actual insertion point is when monitors are rotated in Yosemite. (I had this issue with another monitor and though it was a problem with the monitor, but now I know that's not the case.) So, I tried setting the Rotation setting to 180° degrees again, and the problem still persists.

It doesn't appear that the OP of the above thread ever found a solution. If he did, he didn't post it. Does anyone have any idea what else I could try? Does anyone else remember the days before Jonny Ives took over OS development and Apple products "just worked" as advertised?