Fix desync of online_player_dropdown._player_names

Fixes: #410

exp_scenario/module/gui/elements.lua

@@ -7,34 +7,57 @@ local Gui = require("modules/exp_gui")
 --- @class ExpGui_Elements
 local Elements = {}
 
---- To help with caching and avoid context changes the player list from the previous update is remembered
---- @type (string?)[]
-local _player_names = {}
-
 --- Dropdown which allows selecting an online player
 --- @class ExpGui_Elements.online_player_dropdown: ExpElement
 --- @overload fun(parent: LuaGuiElement): LuaGuiElement
 Elements.online_player_dropdown = Gui.define("player_dropdown")
     :track_all_elements()
     :draw(function(def, parent)
+        local player_names = Elements.online_player_dropdown._access_player_names()
         return parent.add{
             type = "drop-down",
-            items = _player_names,
+            items = player_names,
-            selected_index = #_player_names > 0 and 1 or nil,
+            selected_index = 1,
         }
     end)
     :style{
         height = 24,
     } --[[ @as any ]]
 
+--- To help with caching and avoid context changes the player list from the previous update is remembered
+--- @type (string?)[]
+do local _player_names = {}
+    --- Updates the player name list after a join or leave
+    --- @return (string?)[]
+    function Elements.online_player_dropdown._update_player_names()
+        _player_names[#_player_names] = nil -- Nil last element to account for player leave
+        for i, player in pairs(game.connected_players) do
+            _player_names[i] = player.name
+        end
+        return _player_names
+    end
+
+    --- Safely access the player name list
+    --- @return (string?)[]
+    function Elements.online_player_dropdown._access_player_names()
+        if not _player_names[1] then
+            for i, player in pairs(game.connected_players) do
+                _player_names[i] = player.name
+            end
+        end
+        return _player_names
+    end
+end
+
 --- Get the selected player name from a online player dropdown
 --- @param online_player_dropdown LuaGuiElement
 --- @return string
 function Elements.online_player_dropdown.get_selected_name(online_player_dropdown)
-    local name = _player_names[online_player_dropdown.selected_index]
+    local player_names = Elements.online_player_dropdown._access_player_names()
+    local name = player_names[online_player_dropdown.selected_index]
     if not name then
         online_player_dropdown.selected_index = 1
-        name = _player_names[1] --- @cast name -nil
+        name = player_names[1] --- @cast name -nil
     end
     return name
 end
@@ -43,10 +66,11 @@ end
 --- @param online_player_dropdown LuaGuiElement
 --- @return LuaPlayer
 function Elements.online_player_dropdown.get_selected(online_player_dropdown)
-    local name = _player_names[online_player_dropdown.selected_index]
+    local player_names = Elements.online_player_dropdown._access_player_names()
+    local name = player_names[online_player_dropdown.selected_index]
     if not name then
         online_player_dropdown.selected_index = 1
-        name = _player_names[1] --- @cast name -nil
+        name = player_names[1] --- @cast name -nil
     end
     return assert(game.get_player(name))
 end
@@ -55,20 +79,15 @@ end
 --- Get the number of players in the dropdown
 --- @return number
 function Elements.online_player_dropdown.get_player_count()
-    return #_player_names
+    return #Elements.online_player_dropdown._access_player_names()
 end
 
 --- Update all player dropdowns to match the currently online players
 --- We don't split join and leave because the order would be inconsistent between players and cause desyncs
 function Elements.online_player_dropdown.refresh_online()
-    _player_names[#_player_names] = nil -- Nil last element to account for player leave
+    local player_names = Elements.online_player_dropdown._update_player_names()
-
-    for i, player in pairs(game.connected_players) do
-        _player_names[i] = player.name
-    end
-
     for _, online_player_dropdown in Elements.online_player_dropdown:online_elements() do
-        online_player_dropdown.items = _player_names
+        online_player_dropdown.items = player_names
     end
 end