Adobe InDesign calls this gradient to transparency effect a "gradient feather." Gradient Feather Effect. You apply a gradient feather to a filled object or an image from the effects panel.

In macOS Tahoe Beta 2, Apple included a new option to add a background to the menu bar, making it possible to have a menu bar ...

WinRoll does a lot more than roll windows into their title bar, however—you can turn them transparent, minimize them to the tray, or even toggle the always on top flag.