Use this Trick to Screengrab Disappearing Elements on MacOS
Sometimes elements you want to screen capture on a Mac disappear when you invoke the screenshot shortcut (⌘ + SHIFT + 3 or ⌘ + SHIFT + 4). The solution is to invoke the ⌘ + SHIFT portion first, then trigger the element and then press 3 or 4 while still holding ⌘ + SHIFT and hovering the element.