I've got two monitors side-by-side, one in landscape, and the other in portrait mode. For the portrait monitor, I would love to be able to snap windows to the bottom, middle, and top thirds of the screen. Halves is too much vertical screen space for most things I work on. Is there an application to customize the snap behavior to that extent?
Ubuntu 17.10
Have a look at the gnome extension "gTile".
From the readme:
When you click on the gTile icon it will open a dialog that lets you draw a region on an e. g.
1x3
grid. Once you have drawn the region gTile will move and resize the currently active windows to match that region on the real screen.You can configure a hot key (for instance ctrl+alt+1) to send the current window to a specific field in an
NxM
grid. In your case it could be the first field in a1x3
grid. Likewise for the other two windows.