A truly and uniquely mountain bike specific invention. The first incarnation, the Hite Rite  from Joe Breeze, usually required you to slow down and fiddle with the seatpost quick release - all without putting a foot on the ground. Soon came remote cable activation for the Hite Rite from
, and only much later did internal pneumatic dropper posts come to the market.
How did we live without this for so many years?
Who was first? As with most inventions, it's a matter of patents and sales. For example, Kind Shock  developed a prototype post from an office chair pneumatic lift, but waited 12 years to develop it to market.1) So, it doesn't count - but it's very close.
Patents in this case aren't the best indicators of “practical inventions,” as many adjustable height seats were patented but none came to market, or they were excellent ideas which never proved to be practical, either because they couldn't be developed or because they couldn't sell them.
So, first to market and sales wins. Even if they later lose a patent infringement lawsuit.
Gravity Dropper  is acknowledged as the first dropper post and was quickly followed a year or two later by Maverick, RockShox, Crank Bros., etc. Why not Hite Rite? Because it was frustrating to use while moving and would easily get out of adjustment. But it remains a huge innovation from the man who “invented” the first mountain bike eight years prior.
-  Angell & Breeze : Hite Rite google patents
-  Showa Group : google patents
-  R. J. DeFreitas : google patents
1984 Hite Rite with a 1988 IRD Remote dirtscrolls
2003 Gravity Dropper gravitydropper.com (Oct 2003 archive)
2015 Magura Vyron pinkbike
Electronic dropper post.
2019 RockShox Reverb AXS ebike-mtb
Now, with Bluetooth and an IPhone App