Vim popup (#3817)

* implement vim popups for preview

Details on implementation
-------------------------
- we make use of the |popupwin| api
- we split implementations (Nvim* vs. Vim* prefix) and call the right
  one based on has('nvim')
- we follow a similar structure in each function, using the relevant API
  - popup_list, win_execute, popup_settext in VimShow
  - popup_create in VimCreate
  - popup_close in VimClose

Some differences
----------------
- we DON'T have VimPrepareWindowContent because we use arguments to
  popup_create for borders, padding, etc., and it also takes care of
  buffer creation.
- we follow the protocol of setting and using w:preview for information,
  but we only need the ID
- InsertEnter is the only autocommand required, because of
  popup_create's moved argument. Any cursor movement with 'any' will
  close the popup. This in turns means VimClose is only called from
  InsertMode, so no mode-restoration necessary
- we don't tweak too much in the buffer because vim's popup buffers
  already have most relevant settings and aren't editable without
  calling popup functions.
- I enabled scrollbars, close buttons, dragging, and resizing
- vim popups get as big as they need to by default, so no worrying about
  truncating/hiding/size

Note: we might want to consider changing w:preview to w:ale_preview to
avoid clashes if someone else tries to use the same variable

* floating window: document that vim supports it

* lint: fix indent/cont. lines

3 files changed, 97 insertions, 19 deletions

diff --git a/README.md b/README.md
index e0934694..697f2d87 100644
--- a/README.md
+++ b/README.md
@@ -930,6 +930,13 @@ make it look nicer.
 let g:ale_floating_window_border = ['│', '─', '╭', '╮', '╯', '╰']
 ```
 
+Since vim's default uses nice unicode characters when possible, you can trick
+ale into using that default with
+
+```vim
+let g:ale_floating_window_border = repeat([''], 6)
+```
+
 <a name="faq-vim-lsp"></a>
 
 ### 5.xxii. How can I use ALE and vim-lsp together?
diff --git a/autoload/ale/floating_preview.vim b/autoload/ale/floating_preview.vim
index 729e04b4..f0bc8f80 100644
--- a/autoload/ale/floating_preview.vim
+++ b/autoload/ale/floating_preview.vim
@@ -1,26 +1,35 @@
 " Author: Jan-Grimo Sobez <jan-grimo.sobez@phys.chem.ethz.ch>
 " Author: Kevin Clark <kevin.clark@gmail.com>
+" Author: D. Ben Knoble <ben.knoble+github@gmail.com>
 " Description: Floating preview window for showing whatever information in.
 
-" Precondition: exists('*nvim_open_win')
+" Precondition: exists('*nvim_open_win') || has('popupwin')
 
 function! ale#floating_preview#Show(lines, ...) abort
-    if !exists('*nvim_open_win')
+    if !exists('*nvim_open_win') && !has('popupwin')
         execute 'echom ''Floating windows not supported in this vim instance.'''
 
         return
     endif
 
+    let l:options = get(a:000, 0, {})
+
+    if has('nvim')
+        call s:NvimShow(a:lines, l:options)
+    else
+        call s:VimShow(a:lines, l:options)
+    endif
+endfunction
+
+function! s:NvimShow(lines, options) abort
     " Remove the close autocmd so it doesn't happen mid update
     augroup ale_floating_preview_window
         autocmd!
     augroup END
 
-    let l:options = get(a:000, 0, {})
-
     " Only create a new window if we need it
     if !exists('w:preview') || index(nvim_list_wins(), w:preview['id']) is# -1
-        call s:Create(l:options)
+        call s:NvimCreate(a:options)
     else
         call nvim_buf_set_option(w:preview['buffer'], 'modifiable', v:true)
     endif
@@ -30,7 +39,7 @@ function! ale#floating_preview#Show(lines, ...) abort
 
     call nvim_set_current_win(w:preview['id'])
 
-    for l:command in get(l:options, 'commands', [])
+    for l:command in get(a:options, 'commands', [])
         call execute(l:command)
     endfor
 
@@ -41,13 +50,13 @@ function! ale#floating_preview#Show(lines, ...) abort
         autocmd!
 
         if g:ale_close_preview_on_insert
-            autocmd CursorMoved,TabLeave,WinLeave,InsertEnter <buffer> ++once call s:Close()
+            autocmd CursorMoved,TabLeave,WinLeave,InsertEnter <buffer> ++once call s:NvimClose()
         else
-            autocmd CursorMoved,TabLeave,WinLeave <buffer> ++once call s:Close()
+            autocmd CursorMoved,TabLeave,WinLeave <buffer> ++once call s:NvimClose()
         endif
     augroup END
 
-    let [l:lines, l:width, l:height] = s:PrepareWindowContent(a:lines)
+    let [l:lines, l:width, l:height] = s:NvimPrepareWindowContent(a:lines)
 
     call nvim_win_set_width(w:preview['id'], l:width)
     call nvim_win_set_height(w:preview['id'], l:height)
@@ -56,7 +65,33 @@ function! ale#floating_preview#Show(lines, ...) abort
     call nvim_buf_set_option(w:preview['buffer'], 'modifiable', v:false)
 endfunction
 
-function! s:PrepareWindowContent(lines) abort
+function! s:VimShow(lines, options) abort
+    if g:ale_close_preview_on_insert
+        " Remove the close autocmd so it doesn't happen mid update
+        silent! autocmd! ale_floating_preview_window
+    endif
+
+    " Only create a new window if we need it
+    if !exists('w:preview') || index(popup_list(), w:preview['id']) is# -1
+        call s:VimCreate(a:options)
+    endif
+
+    " Execute commands in window context
+    for l:command in get(a:options, 'commands', [])
+        call win_execute(w:preview['id'], l:command)
+    endfor
+
+    call popup_settext(w:preview['id'], a:lines)
+
+    if g:ale_close_preview_on_insert
+        augroup ale_floating_preview_window
+            autocmd!
+            autocmd InsertEnter * ++once call s:VimClose()
+        augroup END
+    endif
+endfunction
+
+function! s:NvimPrepareWindowContent(lines) abort
     let l:max_height = 10
 
     let l:width = max(map(copy(a:lines), 'strdisplaywidth(v:val)'))
@@ -94,7 +129,7 @@ function! s:PrepareWindowContent(lines) abort
     return [l:lines, l:width, l:height]
 endfunction
 
-function! s:Create(options) abort
+function! s:NvimCreate(options) abort
     let l:buffer = nvim_create_buf(v:false, v:false)
     let l:winid = nvim_open_win(l:buffer, v:false, {
     \ 'relative': 'cursor',
@@ -112,7 +147,32 @@ function! s:Create(options) abort
     let w:preview = {'id': l:winid, 'buffer': l:buffer}
 endfunction
 
-function! s:Close() abort
+function! s:VimCreate(options) abort
+    let l:popup_id = popup_create([], {
+    \    'line': 'cursor+1',
+    \    'col': 'cursor',
+    \    'drag': v:true,
+    \    'resize': v:true,
+    \    'close': 'button',
+    \    'padding': [0, 1, 0, 1],
+    \    'border': [],
+    \    'borderchars': empty(g:ale_floating_window_border) ? [' '] : [
+    \        g:ale_floating_window_border[1],
+    \        g:ale_floating_window_border[0],
+    \        g:ale_floating_window_border[1],
+    \        g:ale_floating_window_border[0],
+    \        g:ale_floating_window_border[2],
+    \        g:ale_floating_window_border[3],
+    \        g:ale_floating_window_border[4],
+    \        g:ale_floating_window_border[5],
+    \    ],
+    \    'moved': 'any',
+    \    })
+    call setbufvar(winbufnr(l:popup_id), '&filetype', get(a:options, 'filetype', 'ale-preview'))
+    let w:preview = {'id': l:popup_id}
+endfunction
+
+function! s:NvimClose() abort
     let l:mode = mode()
     let l:restore_visual = l:mode is# 'v' || l:mode is# 'V' || l:mode is# "\<C-V>"
 
@@ -132,3 +192,12 @@ function! s:Close() abort
         normal! gv
     endif
 endfunction
+
+function! s:VimClose() abort
+    if !exists('w:preview')
+        return
+    endif
+
+    call popup_close(w:preview['id'])
+    unlet w:preview
+endfunction
diff --git a/doc/ale.txt b/doc/ale.txt
index f81bcefb..2d80c322 100644
--- a/doc/ale.txt
+++ b/doc/ale.txt
@@ -651,9 +651,9 @@ problem will be displayed in a balloon instead of hover information.
 Hover information can be displayed in the preview window instead by setting
 |g:ale_hover_to_preview| to `1`.
 
-When using Neovim, if |g:ale_hover_to_floating_preview| or |g:ale_floating_preview|
-is set to 1, the hover information will show in a floating window. And
-|g:ale_floating_window_border| for the border setting.
+When using Neovim or Vim with |popupwin|, if |g:ale_hover_to_floating_preview|
+or |g:ale_floating_preview| is set to 1, the hover information will show in a
+floating window. And |g:ale_floating_window_border| for the border setting.
 
 For Vim 8.1+ terminals, mouse hovering is disabled by default. Enabling
 |balloonexpr| commands in terminals can cause scrolling issues in terminals,
@@ -968,8 +968,8 @@ g:ale_detail_to_floating_preview             *g:ale_detail_to_floating_preview*
   Type: |Number|
   Default: `0`
 
-  When this option is set to `1`, Neovim will use a floating window for
-  ALEDetail output.
+  When this option is set to `1`, Neovim or Vim with |popupwin| will use a
+  floating window for ALEDetail output.
 
 
 g:ale_disable_lsp                                           *g:ale_disable_lsp*
@@ -1200,7 +1200,8 @@ g:ale_floating_preview                                 *g:ale_floating_preview*
   Type: |Number|
   Default: `0`
 
-  When set to `1`, Neovim will use a floating window for ale's preview window.
+  When set to `1`, Neovim or Vim with |popupwin| will use a floating window
+  for ale's preview window.
   This is equivalent to setting |g:ale_hover_to_floating_preview| and
   |g:ale_detail_to_floating_preview| to `1`.
 
@@ -1281,7 +1282,8 @@ g:ale_hover_to_floating_preview               *g:ale_hover_to_floating_preview*
   Type: |Number|
   Default: `0`
 
-  If set to `1`, Neovim will use floating windows for hover messages.
+  If set to `1`, Neovim or Vim with |popupwin| will use floating windows for
+  hover messages.
 
 
 g:ale_keep_list_window_open                       *g:ale_keep_list_window_open*