macOS has an option to show/hide scrollbars based on what you use: mouse or trackpad. If it’s a trackpad, scrollbars only appear during scrolling. If using mouse, scrollbars are visible all the time.
Question: WHY? What is so special about mouse that it requires scrollbars? Or what is so special about trackpad that it allows for scrollbars to be hidden?
UPD: Apple Mouse, despite being a mouse and being called a mouse, works as a trackpad when Automatic option is enabled.