I see the same in Safari and Firefox, (macOS) this is also referenced in the post above:
If I’m understanding this feature correctly, I should be able to open that menu by clicking or pressing = and then navigate with something (arrow keys?) and press something (enter?) to navigate to the highlighted item.
In Safari and Firefox on macOS, whether I open that menu with a click or =, I can’t seem to navigate within that menu. Up/down arrow keys move the page up and down, left/right arrow keys don’t visibly do anything.
Tab switches focus to something else, such as a like button on a post, and removes the yellow highlight from the menu. Tabbing in Firefox did cycle through the menu items before I started writing this reply but now doesn’t, a private window was closed between those attempts.
I see these behaviours here on meta and on my own instance updated to efbc2481d8 (except Firefox tabbing successfully, which I only saw here)