Can't horizontally split the screen with GNOME 3

Solution 1:

https://gitlab.gnome.org/GNOME/mutter/issues/579 shows it's a requested feature not yet implemented directly in GNOME3.

However, there's an extension for GNOME, gTile, which gets you there, as illustrated in https://www.addictivetips.com/ubuntu-linux-tips/tiling-windows-in-gnome-shell/ with its Manual Tiling option. Switch to an app then manually redraw its tile for its horizontal slice (top or bottom). Switch to another app, and manually redraw its tile. gTile looks at the grid drawing and scales apps accordingly.

tilingnome also allows you do do this and also offers horizontal layouts.