On default of our code, number items on main menu is 13. All items on the same line in most of case, but some clients change the phrase to their language, or add other items with long phrase. With 13 long items, may it shift to new line.
Because we can't control the length or item phrase, from version 4.3.0, we made new setting: "Number items of main menu" (core.number_of_items_on_main_menu), it define number items on main menu. Default value is 13, if you use long phrase, you can reduce it value.
Relate issue reported: