Fix #757 - Show :ALEDetail messages in a window

autoload/ale/cursor.vim

@@ -125,9 +125,7 @@ function! ale#cursor#ShowCursorDetail() abort
     if !empty(l:loc)
         let l:message = get(l:loc, 'detail', l:loc.text)
 
-        call s:EchoWithShortMess('off', l:message)
-
-        " Set the echo marker, so we can clear it by moving the cursor.
-        let l:info.echoed = 1
+        call ale#preview#Show(split(l:message, "\n"))
+        echo
     endif
 endfunction

autoload/ale/preview.vim

@@ -0,0 +1,18 @@
+" Author: w0rp <devw0rp@gmail.com>
+" Description: Preview windows for showing whatever information in.
+
+" Open a preview window and show some lines in it.
+function! ale#preview#Show(lines) abort
+    silent pedit ALEPreviewWindow
+    wincmd P
+    setlocal modifiable
+    setlocal noreadonly
+    setlocal nobuflisted
+    setlocal filetype=ale-preview
+    setlocal buftype=nofile
+    setlocal bufhidden=wipe
+    :%d
+    call setline(1, a:lines)
+    setlocal nomodifiable
+    setlocal readonly
+endfunction