I accidentally learned how to maximize my windows in just one direction: click a different mouse button.
At least in Gnome, clicking the "maximize window" decoration with the left mouse button expands it to the size of the screen (if the app permits).
However, clicking the same decoration with the right mouse button expands it horizontally, to the width of the screen, and clicking with the middle mouse button maximizes it vertically.
Paired, sequentially (right click, middle click) they act like a single left click. Indeed, they work the way you'd think in all combinations.