KHamburgerMenu

https://www.phoronix.com/..KDE-KHamburgerMenu

It substitutes the menu bar meaning that it will act non- existent when the menu bar is visible. When there is no menu bar or it is hidden, the KHamburgerMenu will take on all of its responsibilities. I mean for this KHamburgerMenu to be used in all applications that have a menu bar but allow to hide it (or have it hidden by default).

https://invent.kde.org/frameworks/..requests/25

Unrelated css hamburger ↓

https://www.w3schools.com/howto/tryit