Make use of vim 8.2+ popup win

Ref #925

Author: autozimu

autoload/LanguageClient.vim

@@ -8,7 +8,8 @@ let s:TYPE = {
 \   'dict':    type({}),
 \   'funcref': type(function('call'))
 \ }
-let s:FLOAT_WINDOW_AVAILABLE = has('nvim') && exists('*nvim_open_win')
+let s:FLOAT_WINDOW_AVAILABLE = exists('*nvim_open_win')
+let s:POPUP_WINDOW_AVAILABLE = exists('*popup_atcursor')
 
 function! s:AddPrefix(message) abort
     return '[LC] ' . a:message
@@ -268,8 +269,8 @@ function! s:GetVar(...) abort
 endfunction
 
 function! s:ShouldUseFloatWindow() abort
-    let use = s:GetVar('LanguageClient_useFloatingHover')
-    return s:FLOAT_WINDOW_AVAILABLE && (use || use is v:null)
+    let floatingHoverEnabled = s:GetVar('LanguageClient_useFloatingHover', v:true)
+    return s:FLOAT_WINDOW_AVAILABLE && floatingHoverEnabled
 endfunction
 
 function! s:CloseFloatingHover() abort
@@ -321,14 +322,23 @@ endfunction
 
 " Open preview window. Window is open in:
 "   - Floating window on Neovim (0.4.0 or later)
+"   - popup window on vim (8.2 or later)
 "   - Preview window on Neovim (0.3.0 or earlier) or Vim
 function! s:OpenHoverPreview(bufname, lines, filetype) abort
     " Use local variable since parameter is not modifiable
     let lines = a:lines
     let bufnr = bufnr('%')
 
-    let use_float_win = s:ShouldUseFloatWindow()
-    if use_float_win
+    let display_approach = ''
+    if s:ShouldUseFloatWindow()
+        let display_approach = 'float_win'
+    elseif s:POPUP_WINDOW_AVAILABLE && s:GetVar('LanguageClient_usePopupHover', v:true)
+        let display_approach = 'popup_win'
+    else
+        let display_approach = 'preview'
+    endif
+
+    if display_approach ==# 'float_win'
         " When a language server takes a while to initialize and the user
         " calls hover multiple times during that time (for example, via an
         " automatic hover on cursor move setup), we will get a number of
@@ -391,23 +401,30 @@ function! s:OpenHoverPreview(bufname, lines, filetype) abort
         execute 'noswapfile edit!' a:bufname
 
         setlocal winhl=Normal:CursorLine
-    else
+    elseif display_approach ==# 'popup_win'
+        let pop_win_id = popup_atcursor(a:lines, {})
+        call setbufvar(winbufnr(pop_win_id), '&filetype', a:filetype)
+    elseif display_approach ==# 'preview'
         execute 'silent! noswapfile pedit!' a:bufname
         wincmd P
+    else
+        call s:Echoerr('Unknown display approach: ' . display_approach)
     endif
 
-    setlocal buftype=nofile nobuflisted bufhidden=wipe nonumber norelativenumber signcolumn=no modifiable
+    if display_approach !=# 'popup_win'
+        setlocal buftype=nofile nobuflisted bufhidden=wipe nonumber norelativenumber signcolumn=no modifiable
 
-    if a:filetype isnot v:null
-        let &filetype = a:filetype
-    endif
+        if a:filetype isnot v:null
+            let &filetype = a:filetype
+        endif
 
-    call setline(1, lines)
-    setlocal nomodified nomodifiable
+        call setline(1, lines)
+        setlocal nomodified nomodifiable
 
-    wincmd p
+        wincmd p
+    endif
 
-    if use_float_win
+    if display_approach ==# 'float_win'
         " Unlike preview window, :pclose does not close window. Instead, close
         " hover window automatically when cursor is moved.
         let call_after_move = printf('<SID>CloseFloatingHoverOnCursorMove(%s)', string(pos))

doc/LanguageClient.txt

@@ -352,11 +352,23 @@ Valid Options: 1 | 0
 When the value is set to 1, |LanguageClient#textDocument_hover()| opens
 documentation in a floating window instead of preview.
 This variable is effective only when the floating window feature is
-supported.
+supported. Check by running >
+    :echomsg exists('nvim_open_win')
 
 Default: 1 when a floating window is supported, otherwise 0
 Valid Options: 1 | 0
 
+2.26 g:LanguageClient_usePopupHover           *g:LanguageClient_usePopupHover*
+
+When the value is set to 1, |LanguageClient#textDocument_hover()| opens
+documentation in a popup window instead of preview.
+This variable is effective only when the popup window feature is
+supported, which is supported only in vim 8.2+. Check by running >
+    :echomsg has('popupwin')
+
+Default: 1 when a popup window is supported, otherwise 0
+Valid Options: 1 | 0
+
 2.27 g:LanguageClient_virtualTextPrefix     *g:LanguageClient_virtualTextPrefix*
 
 When the value is set to a valid string and |g:LanguageClient_useVirtualText| is