Fix input forwarding: default to XInput, make Steam Input opt-in

Making Steam Input the default backend silently broke input forwarding. Merely
initializing Steam Input activates Steam's in-process XInput interception, which
hides controllers from XInputGetState unless they're bound to our action set for
the running appid. With no such binding (the normal case for a donor appid) Steam
Input reports zero controllers AND XInput now sees nothing -> no input at all.

Reproduced with coop_steam_input_probe: without Steam, XInput slot 0 is seen;
with Steam Input initialized, 0 Steam controllers and the XInput fallback goes
empty.

Default to XInput (RPT delivers guest pads there and it works) and make Steam
Input an opt-in Controllers-panel toggle that switches the backend at runtime;
turning it off restores XInput. All 6 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

host/src/controllers_panel.cpp

@@ -75,6 +75,21 @@ void ControllersPanel::draw(const InputSource& input, const HookStatusView& stat
 	ImGui::TextDisabled("This window is what Remote Play Together captures.");
 	ImGui::TextDisabled("F1: hide overlay (clean mirror)   Esc: quit");
 
+#ifdef COOP_WITH_STEAM
+	ImGui::Checkbox("Use Steam Input (experimental)", &steam_requested_);
+	if (steam_requested_)
+	{
+		ImGui::SameLine();
+		ImGui::TextColored(steam_active_ ? kGreen : kGrey, steam_active_ ? "(active)" : "(starting...)");
+		ImGui::TextDisabled("Needs a controller bound to Steam Input for this app; otherwise");
+		ImGui::TextDisabled("XInput is hidden and no input arrives. Leave off for plain XInput.");
+	}
+	if (steam_note_[0] != '\0')
+	{
+		ImGui::TextColored(kGrey, "%s", steam_note_);
+	}
+#endif
+
 	// --- Guest pads the host receives from RPT -----------------------------
 	ImGui::SeparatorText("Incoming (host receives)");
 	const auto& pads = input.pads();

host/src/controllers_panel.hpp

@@ -21,11 +21,39 @@ public:
 	// reveals the raw axis values and the per-slot poll-rate table.
 	void draw(const InputSource& input, const HookStatusView& status, bool debug_details);
 
+#ifdef COOP_WITH_STEAM
+	// Whether the operator has opted into Steam Input. It's off by default: simply
+	// initializing Steam Input activates Steam's in-process XInput interception,
+	// which hides controllers from XInput unless they're bound to our action set for
+	// this app -- so it can silently break the (working) XInput path. main reconciles
+	// this against the actual backend each frame.
+	[[nodiscard]] bool steam_input_requested() const
+	{
+		return steam_requested_;
+	}
+	void set_steam_active(bool active)
+	{
+		steam_active_ = active;
+	}
+	void on_steam_init_failed()
+	{
+		steam_requested_ = false;
+		steam_active_ = false;
+		steam_note_ = "Steam Input failed to start; using XInput.";
+	}
+#endif
+
 private:
 	// Sampled to turn the hook's cumulative per-slot counters into poll rates.
 	unsigned long long last_state_count_[kMaxPads] = {};
 	double state_rate_[kMaxPads] = {};
 	double last_sample_time_ = 0.0;
+
+#ifdef COOP_WITH_STEAM
+	bool steam_requested_ = false; // operator opted into Steam Input (default off)
+	bool steam_active_ = false;    // Steam Input actually initialized
+	const char* steam_note_ = "";
+#endif
 };
 
 } // namespace coop

host/src/main.cpp

@@ -86,17 +86,16 @@ int run()
 		return 1;
 	}
 
-	// Steam Input is the plan's primary guest-input path (it falls back to XInput
-	// per slot, and to pure XInput if Steam isn't available). When the host is built
-	// without the Steamworks SDK, use XInput directly.
+	// XInput is the default guest-input path: Remote Play Together delivers guest
+	// pads as XInput, and it Just Works. Steam Input is opt-in (Controllers panel) --
+	// merely initializing it activates Steam's in-process XInput interception, which
+	// hides controllers from XInput unless they're bound to our action set for this
+	// app, so making it the default can silently break input. We switch the active
+	// backend at runtime to match the toggle.
+	coop::XInputSource xinput;
+	coop::InputSource* input = &xinput;
 #ifdef COOP_WITH_STEAM
-	std::unique_ptr<coop::InputSource> input = [] {
-		auto steam = std::make_unique<coop::SteamInputSource>();
-		steam->init(steam_manifest_path());
-		return steam;
-	}();
-#else
-	std::unique_ptr<coop::InputSource> input = std::make_unique<coop::XInputSource>();
+	std::unique_ptr<coop::SteamInputSource> steam;
 #endif
 	coop::ControllersPanel controllers;
 	coop::InjectionPanel injection;
@@ -119,6 +118,31 @@ int run()
 
 	while (window.pump_messages())
 	{
+#ifdef COOP_WITH_STEAM
+		// Switch the active input backend to match the Controllers-panel toggle.
+		const bool want_steam = controllers.steam_input_requested();
+		if (want_steam && steam == nullptr)
+		{
+			steam = std::make_unique<coop::SteamInputSource>();
+			if (steam->init(steam_manifest_path()))
+			{
+				input = steam.get();
+				controllers.set_steam_active(true);
+			}
+			else
+			{
+				steam.reset();
+				controllers.on_steam_init_failed();
+			}
+		}
+		else if (!want_steam && steam != nullptr)
+		{
+			steam->shutdown();
+			steam.reset();
+			input = &xinput;
+			controllers.set_steam_active(false);
+		}
+#endif
 		input->poll();
 		injection.publish(input->pads());
 		const HWND game = injection.game_hwnd();