Work around broken event pushing on Tiger

Pushing events from a background thread on Tiger + SDL1
does not appear to do anything at all.

I don't know if the issue is limited to Tiger,
so apply the workaround on all macOS when using SDL1.

Source/interfac.cpp

@@ -37,6 +37,23 @@ namespace devilution {
 
 namespace {
 
+#if defined(__APPLE__) && defined(USE_SDL1)
+// On Tiger PPC, SDL_PushEvent from a background thread appears to do nothing.
+#define SDL_PUSH_EVENT_BG_THREAD_WORKS 0
+#else
+#define SDL_PUSH_EVENT_BG_THREAD_WORKS 1
+#endif
+
+#if !SDL_PUSH_EVENT_BG_THREAD_WORKS
+// This workaround is not completely thread-safe but the worst
+// that can happen is we miss some WM_PROGRESS events,
+// which is not a problem.
+struct {
+	std::atomic<int> type;
+	std::string error;
+} NextCustomEvent;
+#endif
+
 constexpr uint32_t MaxProgress = 534;
 constexpr uint32_t ProgressStepSize = 23;
 
@@ -391,16 +408,31 @@ void DoLoad(interface_mode uMsg)
 	}
 
 	if (!loadResult.has_value()) {
+#if SDL_PUSH_EVENT_BG_THREAD_WORKS
 		SDL_Event event;
 		event.type = CustomEventToSdlEvent(WM_ERROR);
 		event.user.data1 = new std::string(std::move(loadResult).error());
-		SDL_PushEvent(&event);
+		if (SDL_PushEvent(&event) < 0) {
+			LogError("Failed to send WM_ERROR {}", SDL_GetError());
+			SDL_ClearError();
+		}
+#else
+		NextCustomEvent.error = std::move(loadResult).error();
+		NextCustomEvent.type = static_cast<int>(WM_ERROR);
+#endif
 		return;
 	}
 
+#if SDL_PUSH_EVENT_BG_THREAD_WORKS
 	SDL_Event event;
 	event.type = CustomEventToSdlEvent(WM_DONE);
-	SDL_PushEvent(&event);
+	if (SDL_PushEvent(&event) < 0) {
+		LogError("Failed to send WM_DONE {}", SDL_GetError());
+		SDL_ClearError();
+	}
+#else
+	NextCustomEvent.type = static_cast<int>(WM_DONE);
+#endif
 }
 
 struct {
@@ -561,9 +593,16 @@ void IncProgress(uint32_t steps)
 	if (sgdwProgress > MaxProgress)
 		sgdwProgress = MaxProgress;
 	if (!HeadlessMode && sgdwProgress != prevProgress) {
+#if SDL_PUSH_EVENT_BG_THREAD_WORKS
 		SDL_Event event;
 		event.type = CustomEventToSdlEvent(WM_PROGRESS);
-		SDL_PushEvent(&event);
+		if (SDL_PushEvent(&event) < 0) {
+			LogError("Failed to send WM_PROGRESS {}", SDL_GetError());
+			SDL_ClearError();
+		}
+#else
+		NextCustomEvent.type = static_cast<int>(WM_PROGRESS);
+#endif
 	}
 }
 
@@ -589,6 +628,10 @@ void ShowProgress(interface_mode uMsg)
 	ProgressEventHandlerState.done = false;
 	ProgressEventHandlerState.drawnProgress = 0;
 
+#if !SDL_PUSH_EVENT_BG_THREAD_WORKS
+	NextCustomEvent.type = -1;
+#endif
+
 #ifndef USE_SDL1
 	DeactivateVirtualGamepad();
 	FreeVirtualGamepadTextures();
@@ -623,21 +666,36 @@ void ShowProgress(interface_mode uMsg)
 		LogVerbose("Load thread finished in {}ms", SDL_GetTicks() - start);
 	});
 
+	const auto processEvent = [&](const SDL_Event &event) {
+		CheckShouldSkipRendering();
+		if (event.type != SDL_QUIT) {
+			HandleMessage(event, SDL_GetModState());
+		}
+		if (ProgressEventHandlerState.done) {
+			loadThread.join();
+			return false;
+		}
+		return true;
+	};
+
 	while (true) {
 		CheckShouldSkipRendering();
 		SDL_Event event;
 		// We use the real `SDL_PollEvent` here instead of `FetchEvent`
 		// to process real events rather than the recorded ones in demo mode.
 		while (SDL_PollEvent(&event)) {
-			CheckShouldSkipRendering();
-			if (event.type != SDL_QUIT) {
-				HandleMessage(event, SDL_GetModState());
-			}
-			if (ProgressEventHandlerState.done) {
-				loadThread.join();
-				return;
+			if (!processEvent(event)) return;
+		}
+#if !SDL_PUSH_EVENT_BG_THREAD_WORKS
+		if (const int customEventType = NextCustomEvent.type; customEventType != -1) {
+			event.type = CustomEventToSdlEvent(static_cast<interface_mode>(customEventType));
+			if (static_cast<interface_mode>(customEventType) == static_cast<int>(WM_ERROR)) {
+				event.user.data1 = &NextCustomEvent.error;
 			}
+			NextCustomEvent.type = -1;
+			if (!processEvent(event)) return;
 		}
+#endif
 	}
 }