Do not echo messages to the echo line for mouse movements unless the user says so

autoload/ale/balloon.vim

@@ -15,10 +15,18 @@ function! ale#balloon#MessageForPos(bufnr, lnum, col) abort
     " Show the diagnostics message if found, 'Hover' output otherwise
     if l:index >= 0
         return l:loclist[l:index].text
-    else
+    elseif exists('*balloon_show') || getbufvar(
+    \   a:bufnr,
+    \   'ale_set_balloons_legacy_echo',
+    \   get(g:, 'ale_set_balloons_legacy_echo', 0)
+    \)
+        " Request LSP/tsserver hover information, but only if this version of
+        " Vim supports the balloon_show function, or if we turned a legacy
+        " setting on.
         call ale#hover#Show(a:bufnr, a:lnum, a:col, {'called_from_balloonexpr': 1})
-        return ''
     endif
+
+    return ''
 endfunction
 
 function! ale#balloon#Expr() abort