Fix #272 by checking if quickfix is open before reopening to avoid triggering a BufEnter event that causes quickly repeating linting runs when g:ale_lint_on_enter = 1.

Add test assertions that quickfix window closes when lists become empty again.

autoload/ale/list.vim

@@ -36,10 +36,12 @@ function! ale#list#SetLists(buffer, loclist) abort
     if len(a:loclist) > 0 || g:ale_keep_list_window_open
         let l:winnr = winnr()
 
-        if g:ale_set_quickfix
-            copen
-        elseif g:ale_set_loclist
-            lopen
+        if !ale#list#IsQuickfixOpen()
+          if g:ale_set_quickfix
+              copen
+          elseif g:ale_set_loclist
+              lopen
+          endif
         endif
 
         " If focus changed, restore it (jump to the last window).

test/test_list_opening.vader

@@ -49,6 +49,10 @@ Execute(The quickfix window should open for just the loclist):
   call ale#list#SetLists(bufnr('%'), g:loclist)
   Assert ale#list#IsQuickfixOpen()
 
+  " Clear the list and it should close again.
+  call ale#list#SetLists(bufnr('%'), [])
+  Assert !ale#list#IsQuickfixOpen()
+
 Execute(The quickfix window should stay open for just the loclist):
   let g:ale_open_list = 1
   let g:ale_keep_list_window_open = 1
@@ -76,6 +80,10 @@ Execute(The quickfix window should open for the quickfix list):
   call ale#list#SetLists(bufnr('%'), g:loclist)
   Assert ale#list#IsQuickfixOpen()
 
+  " Clear the list and it should close again.
+  call ale#list#SetLists(bufnr('%'), [])
+  Assert !ale#list#IsQuickfixOpen()
+
 Execute(The quickfix window should stay open for the quickfix list):
   let g:ale_set_quickfix = 1
   let g:ale_open_list = 1