Overload of Vim's default `<C-W>=` normal command that sets the preview window's height to `&previewheight`

custom-wincmd-equals.vim

function! s:CustomWincmdEquals(visual)
    try
        let w = winnr()
        wincmd P
        exe &previewheight.'wincmd _'
        exe w.'wincmd w'
    catch /^E441:/
        continue
    finally
        return "\<C-W>=" . (a:visual ? 'gv' : '')
    endtry
endfunction

nno <silent> <Plug>CustomwincmdEquals @=<SID>CustomWincmdEquals(0)<CR>
vno <silent> <Plug>CustomwincmdEquals @=<SID>CustomWincmdEquals(1)<CR>

nmap <C-W>= <Plug>CustomwincmdEquals
vmap <C-W>= <Plug>CustomwincmdEquals