Octopi UI is missing the hamburger icon (located top right - above the wrench icon)
What did you already try to solve it?
I've tried updating octoprint and plugins. Completely restarting the installation from scratch (for octopi and plugins)
tried safe mode, restarting, rebooting, different browsers, different devices.
That is the Tab Overflow Menu, it's only activated when you have more tabs than what fits. Either you have not installed or have disabled extra plugins.
If you installed that plugin Swapper3D and it's not showing up then there is probably something wrong with the plugin and it's not actually loading properly. Looking at the plugin on GitHub, since it's not officially registered on the repo, it requires a minimum python version of 3.7. If your version of python listed in OctoPrint's interface on the bottom left is lower than that it won't load. Otherwise, look through octoprint.log for errors or open an issue on their GitHub repo for support.