The fully separable zipper was invented in 1927 by Gideon Sundback. It’s covered by patents 1813433 and 1813432.
From the invention of the zipper in 1913 until that point, zippers had been limited in their usage by an attached end. It was suitable for coverall suits, luggage and pullovers. But it made it inconvenient for applications in jackets. The new unlockable/ fully separable bottom end of the zipper allowed for its easy usage on jackets. By 1930, zipper fronted jackets were well on their way to becoming common.

1925. Typical 1920s style. Button front, marketed as a windbreaker, specifically for the sporting set. Suede was extremely common in this era.

1929. Before the modern separable bottom to the zipper, this pullover style was the workaround.

1930. Early zipper front style.