Add F10 back-buffer screenshot (PNG, focus-independent)

Capture the rendered back buffer to a timestamped PNG next to the exe via
WIC, triggered by F10 (delivered even when unfocused). The capture runs in
render_frame just before Present so it includes the ImGui overlay, and reads
off the GPU so it works regardless of window focus, z-order, or occlusion. A
brief toast confirms the save (drawn the next frame, so it's never in the shot).
F10 chosen to avoid Steam's F12; its WM_SYSKEYDOWN is swallowed so Windows
doesn't enter menu mode. Documented in the Help menu + README.

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

host/src/main.cpp

@@ -6,6 +6,8 @@
 // this window via Windows Graphics Capture (Video mirror panel).
 
 #include <cstdint>
+#include <cstdio>
+#include <string>
 
 #include <windows.h>
 
@@ -18,6 +20,7 @@
 #include "audio_panel.hpp"
 #include "capture_panel.hpp"
 #include "controllers_panel.hpp"
+#include "coop/tool_paths.hpp"
 #include "d3d11_window.hpp"
 #include "imgui_layer.hpp"
 #include "inject/mkb_forward.hpp"
@@ -26,13 +29,59 @@
 #include "log_panel.hpp"
 #include "ui/app_chrome.hpp"
 
-#ifdef COOP_WITH_STEAM
-#include <string>
-#endif
-
 namespace
 {
 
+// Timestamped screenshot path next to the exe (e.g. coop_shot_20260622_143501.png).
+std::wstring screenshot_path()
+{
+	SYSTEMTIME st{};
+	GetLocalTime(&st);
+	wchar_t name[64];
+	swprintf(name, static_cast<int>(std::size(name)), L"coop_shot_%04u%02u%02u_%02u%02u%02u.png", st.wYear,
+	         st.wMonth, st.wDay, st.wHour, st.wMinute, st.wSecond);
+	return coop::exe_directory() + name;
+}
+
+// Just the filename of a path, narrowed to UTF-8 for an ImGui confirmation toast.
+std::string screenshot_basename(const std::wstring& path)
+{
+	const std::size_t slash = path.find_last_of(L"\\/");
+	const std::wstring file = slash == std::wstring::npos ? path : path.substr(slash + 1);
+	if (file.empty())
+	{
+		return {};
+	}
+	const int n = WideCharToMultiByte(CP_UTF8, 0, file.c_str(), static_cast<int>(file.size()), nullptr, 0,
+	                                  nullptr, nullptr);
+	std::string out(static_cast<std::size_t>(n), '\0');
+	WideCharToMultiByte(CP_UTF8, 0, file.c_str(), static_cast<int>(file.size()), out.data(), n, nullptr, nullptr);
+	return out;
+}
+
+// Brief fading "saved" confirmation after an F10 screenshot (bottom-left, non-interactive
+// so it never steals a click). It's drawn the frame *after* the capture, so it never lands
+// in the shot itself.
+void draw_screenshot_toast(double seconds_since, const std::string& name)
+{
+	const float fade = 1.0f - static_cast<float>(seconds_since) / 2.5f;
+	if (fade <= 0.0f || name.empty())
+	{
+		return;
+	}
+	const ImGuiViewport* vp = ImGui::GetMainViewport();
+	ImGui::SetNextWindowPos(ImVec2(vp->WorkPos.x + 12.0f, vp->WorkPos.y + vp->WorkSize.y - 44.0f));
+	ImGui::SetNextWindowBgAlpha(0.45f * fade);
+	const ImGuiWindowFlags flags = ImGuiWindowFlags_NoDecoration | ImGuiWindowFlags_NoInputs |
+	                               ImGuiWindowFlags_AlwaysAutoResize | ImGuiWindowFlags_NoSavedSettings |
+	                               ImGuiWindowFlags_NoFocusOnAppearing | ImGuiWindowFlags_NoNav;
+	ImGui::Begin("##shot_toast", nullptr, flags);
+	ImGui::PushStyleColor(ImGuiCol_Text, ImVec4(0.6f, 1.0f, 0.6f, fade));
+	ImGui::Text("Saved screenshot: %s", name.c_str());
+	ImGui::PopStyleColor();
+	ImGui::End();
+}
+
 #ifdef COOP_WITH_STEAM
 // Absolute path to the bundled Steam Input action manifest (next to the exe).
 std::string steam_manifest_path()
@@ -151,6 +200,8 @@ int run()
 	// Play Together; the pipelines keep running underneath either way.
 	bool show_overlay = true;
 	double overlay_hidden_at = 0.0;
+	double last_shot_at = -10.0; // when the last F10 screenshot was saved (for the toast)
+	std::string last_shot_name;
 	coop::UiState ui;
 	// Persist the "Debug details" verbosity in the .ini. Register before the first
 	// begin_frame() below, which is when ImGui loads the .ini and replays our handler.
@@ -210,6 +261,10 @@ int run()
 		{
 			injection.toggle_cursor_release(); // free/clip the operator's mouse for clipping games
 		}
+		if (ImGui::IsKeyPressed(ImGuiKey_F10, false))
+		{
+			window.request_screenshot(screenshot_path()); // captured at Present, overlay included
+		}
 
 		if (show_overlay)
 		{
@@ -234,6 +289,7 @@ int run()
 			{
 				log.draw();
 			}
+			draw_screenshot_toast(ImGui::GetTime() - last_shot_at, last_shot_name);
 		}
 		else
 		{
@@ -265,6 +321,14 @@ int run()
 			    imgui.end_frame();
 		    },
 		    sync_interval);
+
+		// render_frame saves a pending F10 screenshot just before Present; pick up the
+		// result here so next frame shows the confirmation toast (kept out of the shot).
+		if (std::wstring shot = window.take_screenshot_result(); !shot.empty())
+		{
+			last_shot_at = ImGui::GetTime();
+			last_shot_name = screenshot_basename(shot);
+		}
 	}
 
 	return 0;