tests/ui: warn when the correct state was seen temporarily.

Helped-by: Thiago de Arruda <tpadilha84@gmail.com>

test/functional/ui/screen.lua

@@ -230,20 +230,41 @@ end
 
 function Screen:wait(check, timeout)
   local err, checked = false
+  local success_seen = false
+  local failure_after_success = false
   local function notification_cb(method, args)
     assert(method == 'redraw')
     self:_redraw(args)
     err = check()
     checked = true
     if not err then
+      success_seen = true
       stop()
+    elseif success_seen and #args > 0 then
+      failure_after_success = true
+      --print(require('inspect')(args))
     end
+
     return true
   end
   run(nil, notification_cb, nil, timeout or default_screen_timeout)
   if not checked then
     err = check()
   end
+
+  if failure_after_success then
+    print([[
+Warning: Screen changes have been received after the expected state was seen.
+This is probably due to an indeterminism in the test. Try adding
+`wait()` (or even a separate `screen:expect(...)`) at a point of possible
+indeterminism, typically in between a `feed()` or `execute()` which is non-
+synchronous, and a synchronous api call.
+      ]])
+    local tb = debug.traceback()
+    local index = string.find(tb, '\n%s*%[C]')
+    print(string.sub(tb,1,index))
+  end
+
   if err then
     assert(false, err)
   end