Diagnostics echoed and cut-off in gvim 8.2

[zlindenbaum]

• Did you upgrade to latest plugin version? Yep
• Did you upgrade to/compile latest binary? Run shell command
  bin/languageclient --version to get its version number. Yep
• (Neovim users only) Did you check output of :checkhealth LanguageClient? N/A
• Did you check troubleshooting? Yep

Describe the bug

I'm unsure if this is a bug, or intended functionality. I can't seem to get diagnostics to render in a popup window (as in coc.nvim, which works in my vim client, if extremely slowly): they're echoed to the bottom of the page, with the majority of their contents cut-off. My vim client has popup functionality, and I use that for rendering documentation. If it's relevant, I'm running NixOS, and using the vimHugeX package for gvim.

Environment

• neovim/vim version (nvim --version or vim --version): vim 8.2
• This plugin version (git rev-parse --short HEAD): cf6dd11
• This plugin's binary version (bin/languageclient --version): 0.1.161
• Minimal vimrc content (A minimal vimrc is the smallest vimrc that could
  reproduce the issue. Refer to an example here):

call plug#begin('~/.local/share/nvim/plugged')

Plug 'autozimu/LanguageClient-neovim', {
    \ 'branch': 'next',
    \ 'do': 'bash install.sh',
    \ }

call plug#end()

augroup filetype_rust
    autocmd!
    autocmd BufReadPost *.rs setlocal filetype=rust
augroup END

" Always draw sign column. Prevent buffer moving when adding/deleting sign.
set signcolumn=yes

let g:LanguageClient_loggingLevel = 'INFO'
let g:LanguageClient_virtualTextPrefix = ''
let g:LanguageClient_loggingFile =  expand('~/.local/share/nvim/LanguageClient.log')
let g:LanguageClient_serverStderr = expand('~/.local/share/nvim/LanguageServer.log')

set hidden

set rtp+=~/.vim/plugged/LanguageClient-neovim
let g:LanguageClient_serverCommands = { 'haskell': ['haskell-language-server-wrapper', '--lsp'] }
let g:LanguageClient_usePopupHover = 1
let g:LanguageClient_useVirtualText = "No"
let g:LanguageClient_hoverPreview = "always"

set completefunc=LanguageClient#complete
" set omnifunc=LanguageClient#complete

map <Leader>h :call LanguageClient_contextMenu()<CR>
autocmd FileType haskell nnoremap <Esc> :call LanguageClient#textDocument_hover()<CR>

• Language server link and version:
  Haskell Language Server 1.6.1.1