Index: binaries/data/mods/public/gui/loading/loading.js
===================================================================
--- binaries/data/mods/public/gui/loading/loading.js
+++ binaries/data/mods/public/gui/loading/loading.js
@@ -55,25 +55,28 @@
 	}
 
 	Engine.GetGUIObjectByName("progressText").caption = "";
-	Engine.GetGUIObjectByName("progressbar").caption = 0;
+
+	let progressbar = Engine.GetGUIObjectByName("progressbar");
+	progressbar.caption = 0;
+	progressbar.onGameLoadProgress = displayProgress;
 
 	Engine.GetGUIObjectByName("quoteText").caption = translate(pickRandom(Engine.ReadFileLines(g_QuotesFile).filter(line => line)));
 }
 
-function displayProgress()
+function displayProgress(data)
 {
 	// Make the progessbar finish a little early so that the user can actually see it finish
-	if (g_Progress >= 100)
+	if (data.progress >= 100)
 		return;
 
 	// Show 100 when it is really 99
-	let progress = g_Progress + 1;
+	let progress = data.progress + 1;
 
 	Engine.GetGUIObjectByName("progressbar").caption = progress; // display current progress
 	Engine.GetGUIObjectByName("progressText").caption = progress + "%";
 
 	// Displays detailed loading info rather than a percent
-	// Engine.GetGUIObjectByName("progressText").caption = g_LoadDescription; // display current progess details
+	// Engine.GetGUIObjectByName("progressText").caption = data.description; // display current progess details
 
 	// Keep curved right edge of progress bar in sync with the rest of the progress bar
 	let middle = Engine.GetGUIObjectByName("progressbar");
Index: binaries/data/mods/public/gui/loading/loading.xml
===================================================================
--- binaries/data/mods/public/gui/loading/loading.xml
+++ binaries/data/mods/public/gui/loading/loading.xml
@@ -21,7 +21,6 @@
 			>
 				<object name="progressbar_left" size="-8 0 8 100%" type="image" sprite="LoadingProgressBarLeft"/>
 				<object name="progressbar_right" size="8 0 24 100%" type="image" sprite="LoadingProgressBarRight"/>
-				<action on="Progress">displayProgress();</action>
 				<object name="progressText" size="0 0 100% 100%" type="text" style="LoadingBarText"/>
 			</object>
 		</object>
Index: binaries/data/mods/public/gui/session/messages.js
===================================================================
--- binaries/data/mods/public/gui/session/messages.js
+++ binaries/data/mods/public/gui/session/messages.js
@@ -729,11 +729,13 @@
 	);
 }
 
-function onReplayOutOfSync()
+function onReplayOutOfSync(msg)
 {
 	messageBox(
 		500, 140,
-		translate("Out-Of-Sync error!") + "\n" +
+		sprintf(translate("Out-Of-Sync error on turn %(turn)s."), {
+			"turn": msg.turn
+		}) + "\n" +
 			// Translation: This is shown if replay is out of sync
 			translateWithContext("Out-Of-Sync", "The current game state is different from the original game state."),
 		translate("Out of Sync")
Index: binaries/data/mods/public/gui/session/session.xml
===================================================================
--- binaries/data/mods/public/gui/session/session.xml
+++ binaries/data/mods/public/gui/session/session.xml
@@ -24,7 +24,7 @@
 	</action>
 
 	<action on="ReplayOutOfSync">
-		onReplayOutOfSync();
+		onReplayOutOfSync(arguments[0]);
 	</action>
 
 	<!-- Hotkeys won't work properly unless outside menu -->
Index: source/gui/CGUI.h
===================================================================
--- source/gui/CGUI.h
+++ source/gui/CGUI.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2017 Wildfire Games.
+/* Copyright (C) 2019 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -98,6 +98,13 @@
 	void SendEventToAll(const CStr& EventName);
 
 	/**
+	 * Sends a specified script event with a given argument to every object.
+	 *
+	 * @param argument A JS::Value that further describes event data.
+	 */
+	void SendEventToAll(const CStr& EventName, JS::HandleValue argument);
+
+	/**
 	 * Displays the whole GUI
 	 */
 	void Draw();
Index: source/gui/CGUI.cpp
===================================================================
--- source/gui/CGUI.cpp
+++ source/gui/CGUI.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2018 Wildfire Games.
+/* Copyright (C) 2019 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -274,8 +274,13 @@
 	// (sending events here) wasn't converting to lower case,
 	// leading to a similar problem.
 	// now fixed; case is irrelevant since all are converted to lower.
-	GUI<CStr>::RecurseObject(0, m_BaseObject,
-		&IGUIObject::ScriptEvent, EventName.LowerCase());
+
+	GUI<CStr>::RecurseObject(0, m_BaseObject, &IGUIObject::ScriptEvent, EventName.LowerCase());
+}
+
+void CGUI::SendEventToAll(const CStr& EventName, JS::HandleValue argument)
+{
+	GUI<CStr>::RecurseObject(0, m_BaseObject, &IGUIObject::ScriptEvent, EventName.LowerCase(), argument);
 }
 
 CGUI::CGUI(const shared_ptr<ScriptRuntime>& runtime)
Index: source/gui/GUIManager.h
===================================================================
--- source/gui/GUIManager.h
+++ source/gui/GUIManager.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2018 Wildfire Games.
+/* Copyright (C) 2019 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -116,6 +116,7 @@
 	 * See CGUI::SendEventToAll; applies to the currently active page.
 	 */
 	void SendEventToAll(const CStr& eventName) const;
+	void SendEventToAll(const CStr& eventName, JS::HandleValue argument) const;
 
 	/**
 	 * See CGUI::TickObjects; applies to @em all loaded pages.
Index: source/gui/GUIManager.cpp
===================================================================
--- source/gui/GUIManager.cpp
+++ source/gui/GUIManager.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2018 Wildfire Games.
+/* Copyright (C) 2019 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -373,6 +373,11 @@
 	top()->SendEventToAll(eventName);
 }
 
+void CGUIManager::SendEventToAll(const CStr& eventName, JS::HandleValue argument) const
+{
+	top()->SendEventToAll(eventName, argument);
+}
+
 void CGUIManager::TickObjects()
 {
 	PROFILE3("gui tick");
Index: source/gui/GUIutil.h
===================================================================
--- source/gui/GUIutil.h
+++ source/gui/GUIutil.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2015 Wildfire Games.
+/* Copyright (C) 2019 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -222,6 +222,7 @@
 	typedef void (IGUIObject::*void_Object_pFunction_argT)(const T& arg);
 	typedef void (IGUIObject::*void_Object_pFunction_argRefT)(T& arg);
 	typedef void (IGUIObject::*void_Object_pFunction)();
+	typedef void (IGUIObject::*void_Object_pFunction_argTJS)(const T& arg, JS::HandleValue arg2);
 
 	/**
 	 * If you want to call a IGUIObject-function
@@ -257,7 +258,6 @@
 
 		(pObject->*pFunc)(Argument);
 
-		// Iterate children
 		for (IGUIObject* const& obj : *pObject)
 			RecurseObject(RR, obj, pFunc, Argument);
 	}
@@ -274,11 +274,21 @@
 
 		(pObject->*pFunc)(Argument);
 
-		// Iterate children
 		for (IGUIObject* const& obj : *pObject)
 			RecurseObject(RR, obj, pFunc, Argument);
 	}
 
+	static void RecurseObject(int RR, IGUIObject* pObject, void_Object_pFunction_argTJS pFunc, const T& Argument1, JS::HandleValue Argument2)
+	{
+		if (CheckIfRestricted(RR, pObject))
+			return;
+
+		(pObject->*pFunc)(Argument1, Argument2);
+
+		for (IGUIObject* const& obj : *pObject)
+			RecurseObject(RR, obj, pFunc, Argument1, Argument2);
+	}
+
 	/**
 	 * With no argument.
 	 *
Index: source/ps/GameSetup/GameSetup.cpp
===================================================================
--- source/ps/GameSetup/GameSetup.cpp
+++ source/ps/GameSetup/GameSetup.cpp
@@ -183,12 +183,16 @@
 // display progress / description in loading screen
 void GUI_DisplayLoadProgress(int percent, const wchar_t* pending_task)
 {
-	g_GUI->GetActiveGUI()->GetScriptInterface()->SetGlobal("g_Progress", percent, true, false, true);
-	g_GUI->GetActiveGUI()->GetScriptInterface()->SetGlobal("g_LoadDescription", pending_task, true, false, true);
-	g_GUI->GetActiveGUI()->SendEventToAll("progress");
-}
+	const ScriptInterface& scriptInterface = *(g_GUI->GetActiveGUI()->GetScriptInterface());
+	JSContext* cx = scriptInterface.GetContext();
+	JSAutoRequest rq(cx);
 
+	JS::RootedValue eventData(cx, JS::ObjectValue(*JS_NewPlainObject(cx)));
+	scriptInterface.SetProperty(eventData, "progress", percent);
+	scriptInterface.SetProperty(eventData, "description", pending_task);
 
+	g_GUI->GetActiveGUI()->SendEventToAll("GameLoadProgress", eventData);
+}
 
 void Render()
 {
Index: source/simulation2/system/ReplayTurnManager.cpp
===================================================================
--- source/simulation2/system/ReplayTurnManager.cpp
+++ source/simulation2/system/ReplayTurnManager.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2017 Wildfire Games.
+/* Copyright (C) 2019 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -76,12 +76,21 @@
 	ENSURE(m_Simulation2.ComputeStateHash(hash, quickHash));
 	hash = Hexify(hash);
 
-	if (hash != expectedHash)
-	{
-		m_HasSyncError = true;
-		LOGERROR("Replay out of sync on turn %d", turn);
-		g_GUI->SendEventToAll("ReplayOutOfSync");
-	}
+	if (hash == expectedHash)
+		return;
+
+	m_HasSyncError = true;
+	LOGERROR("Replay out of sync on turn %d", turn);
+
+	const ScriptInterface& scriptInterface = m_Simulation2.GetScriptInterface();
+	JSContext* cx = scriptInterface.GetContext();
+	JSAutoRequest rq(cx);
+
+	JS::RootedValue replayData(cx, JS::ObjectValue(*JS_NewPlainObject(cx)));
+	scriptInterface.SetProperty(replayData, "turn", turn);
+	scriptInterface.SetProperty(replayData, "hash", hash);
+	scriptInterface.SetProperty(replayData, "expectedHash", expectedHash);
+	g_GUI->SendEventToAll("ReplayOutOfSync", replayData);
 }
 
 void CReplayTurnManager::DoTurn(u32 turn)