Let's say I have a window layout like below:
[1][2]
[3..3]
And I want to expand window 2 to look like this:
[1][2]
[3][2]
So window 2 will now be full height
You can do this by moving window 2 rightwards (<C-W><S-L>
or :wincmd L
):
To get back to the original arrangement from here, move window 3 downwards: focus on window 3, then <C-W><S-J>
or :wincmd J
.