[fix] MenuSorter: take care of orphaned submenus

See https://github.com/koreader/koreader/pull/3167#issuecomment-326805768

frontend/ui/menusorter.lua

@@ -157,6 +157,13 @@ function MenuSorter:sort(item_table, order)
             v.text = self.orphaned_prefix .. v.text
             -- prevent text being prepended to item on menu reload, i.e., on switching between reader and filemanager
             v.new = true
+            -- deal with orphaned submenus
+            if #v > 0 then
+                v.sub_item_table = {}
+                for i=1,#v do
+                    v.sub_item_table[i] = v[i]
+                end
+            end
         end
         table.insert(menu_table["KOMenu:menu_buttons"][1], v)
     end

spec/unit/menusorter_spec.lua

@@ -80,6 +80,36 @@ describe("MenuSorter module", function()
             assert.is_true(string.sub(menu_item.text,1,string.len(MenuSorter.orphaned_prefix))==MenuSorter.orphaned_prefix)
         end
     end)
+    it("should display submenu of orphaned submenu", function()
+        local menu_items = {
+            ["KOMenu:menu_buttons"] = {},
+            main = {text="Main"},
+            search = {text="Search"},
+            tools = {text="Tools"},
+            setting = {text="Settings"},
+            submenu = {text="Submenu"},
+            submenu_item1 = {text="Submenu item 1"},
+            submenu_item2 = {text="Submenu item 2"},
+        }
+        local order = {
+            ["KOMenu:menu_buttons"] = {
+                "setting",
+            },
+            setting = {},
+            submenu = {
+                "submenu_item2",
+                "submenu_item1",
+            },
+        }
+
+        local test_menu = MenuSorter:sort(menu_items, order)
+
+        -- all four should be in the first menu
+        assert.is_true(#test_menu[1] == 4)
+        assert.is_truthy(test_menu[1][3].sub_item_table)
+        assert.equals(test_menu[1][3].sub_item_table[1].id, "submenu_item2")
+        assert.equals(test_menu[1][3].sub_item_table[2].id, "submenu_item1")
+    end)
     it("should not treat disabled as orphans", function()
         local menu_items = {
             ["KOMenu:menu_buttons"] = {},