公司的 MacBook Pro (13-inch, 2017, Two Thunderbolt 3 ports) 透過 HDMI 接 Dell P2419H 一直都有 Underscan 的問題：
出自「About overscan and underscan on your Mac, Apple TV, or other display」
本來想透過 Underscan slide 修改 (像是下面這張圖)，但發現系統內沒有 Underscan slide。
出自「About overscan and underscan on your Mac, Apple TV, or other display」
找了不少文章後後來是在「Fixing Issues with Overscan/Underscan(Black Borders) on macOS」這篇的 comment 看到解法：
Ran into this same underscan problem with black borders showing up on my new Dell 24-inch Ultrasharp U2415 connected to a 2013 Macbook Air running High Sierra, and after hours of looking into the problem the fix turned out to be super simple:
Just restart in Safe Mode.
That’s it. Restart your Mac in Safe Mode (restart, hold down the Shift key after the Apple BONG sounds, then restart normally once again for good measure. I don’t know what this clears or resets but it worked for me. No more letterboxing or black borders with the native 1920×1200 resolution selected.
So maybe something to try first for anyone coming across this post.
照著重開進 Safe mode 後再開回一般模式就正常了 (what？？？)，先記錄起來，讓我之後遇到時可以搜尋到自己的文章...