Hi, I just stumbled on the same issue and finally managed to solve it using:
set winheight=5
set winminheight=5
autocmd WinEnter * wincmd _
this maximizes the horizontal split when entering the window while respecting the winminheight. you could also do e.g.
set winwidth=80
set winminwidth=80
autocmd WinEnter * wincmd |
which does the same for vsplit, but it doesn’t behave exactly as I’d want since it maximizes the window even when not necessary. it would be nice if there was sth like winmaxwidth.