Make the host per-monitor DPI aware and scale the ImGui overlay

The window sized itself to GetSystemMetrics(SM_CXSCREEN/CYSCREEN) but the
process was DPI-unaware, so on a scaled display (e.g. 4K @ 150%) Windows
handed us a virtualized resolution and bitmap-stretched the whole window up
to native -- softening the mirror, which is the tool's entire point.

Declare per-monitor-v2 awareness at startup so GetSystemMetrics/GetDpiForWindow
report true pixels. That alone would shrink the fixed-13px ImGui overlay to
crisp-but-tiny, so pair it with UI scaling: rebuild the default-font atlas at a
DPI-scaled SizePixels (crisp at the target size, unlike FontGlobalScale's
bitmap stretch) and ScaleAllSizes() the style. Net: same physical size as
before, now sharp.

- common/include/coop/dpi.hpp: pure DPI->scale math (uses USER_DEFAULT_SCREEN_DPI
  and a named kBaseFontPx, not bare 96/13 literals), with a zero fallback and
  clamping. Unit-tested by tests/dpi_test.cpp.
- imgui_layer: apply_dpi() at init from GetDpiForWindow; set_dpi() for runtime
  changes (rebuild atlas + reset/scale style + invalidate the DX11 font texture).
- d3d11_window: latch WM_DPICHANGED (honor the suggested rect), expose
  take_dpi_change(); main loop polls it and calls imgui.set_dpi() between frames.

The DPI math is unit-tested; the actual awareness + font rasterization + live
WM_DPICHANGED rescale are verified by hand.

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

host/src/d3d11_window.cpp

@@ -334,6 +334,22 @@ LRESULT CALLBACK D3D11Window::wnd_proc(HWND hwnd, UINT msg, WPARAM wparam, LPARA
 			self->resize_height_ = HIWORD(lparam);
 		}
 		return 0;
+	case WM_DPICHANGED:
+		// Per-monitor-v2: the DPI of the display we're on changed. Resize to the rect Windows suggests
+		// in lparam (its recommended handling; the resulting WM_SIZE repaints the swap chain via the
+		// deferred-resize path above), then latch the new DPI for the overlay to rescale its font/style.
+		if (self != nullptr)
+		{
+			if (const auto* suggested = reinterpret_cast<const RECT*>(lparam); suggested != nullptr)
+			{
+				SetWindowPos(hwnd, nullptr, suggested->left, suggested->top,
+				             suggested->right - suggested->left, suggested->bottom - suggested->top,
+				             SWP_NOZORDER | SWP_NOACTIVATE);
+			}
+			self->dpi_pending_ = true;
+			self->pending_dpi_ = HIWORD(wparam); // X and Y DPI are equal; HIWORD is the Y value
+		}
+		return 0;
 	case WM_SYSKEYDOWN:
 		// F10 is our screenshot key; ImGui already saw this message (handler runs above),
 		// so swallow it here to stop DefWindowProc from flicking into Win32 menu mode.

host/src/d3d11_window.hpp

@@ -60,6 +60,21 @@ public:
 		return device_lost_reason_;
 	}
 
+	// If a WM_DPICHANGED arrived since the last call (the window moved to a different-DPI monitor, or
+	// the display scale changed at runtime), returns true and writes that monitor's DPI to `dpi`,
+	// clearing the pending flag; otherwise returns false. The overlay layer polls this to rescale its
+	// fonts/style. One-shot, mirroring the deferred-resize handling in pump_messages().
+	[[nodiscard]] bool take_dpi_change(unsigned& dpi)
+	{
+		if (!dpi_pending_)
+		{
+			return false;
+		}
+		dpi = pending_dpi_;
+		dpi_pending_ = false;
+		return true;
+	}
+
 	[[nodiscard]] HWND hwnd() const
 	{
 		return hwnd_;
@@ -91,6 +106,8 @@ private:
 	bool resize_pending_ = false;
 	UINT resize_width_ = 0;
 	UINT resize_height_ = 0;
+	bool dpi_pending_ = false;    // set by WM_DPICHANGED, consumed by take_dpi_change()
+	unsigned pending_dpi_ = 0;    // the monitor DPI reported alongside that WM_DPICHANGED
 	bool device_lost_ = false;
 	HRESULT device_lost_reason_ = S_OK;
 

host/src/imgui_layer.cpp

@@ -6,6 +6,7 @@
 #include <imgui_impl_dx11.h>
 #include <imgui_impl_win32.h>
 
+#include "coop/dpi.hpp"
 #include "coop/tool_paths.hpp"
 #include "ui/app_chrome.hpp"
 
@@ -54,7 +55,6 @@ bool ImGuiLayer::init(HWND hwnd, ID3D11Device* device, ID3D11DeviceContext* cont
 	ini_path_ = to_utf8(ini_w);
 	io.IniFilename = ini_path_.c_str();
 	set_layout_persisted(had_layout);
-	ImGui::StyleColorsDark();
 
 	if (!ImGui_ImplWin32_Init(hwnd))
 	{
@@ -65,9 +65,49 @@ bool ImGuiLayer::init(HWND hwnd, ID3D11Device* device, ID3D11DeviceContext* cont
 		return false;
 	}
 	initialized_ = true;
+
+	// Scale the overlay (font + style) to the monitor the window opened on. The process is
+	// per-monitor-DPI-aware (set in wWinMain), so GetDpiForWindow returns that monitor's real DPI.
+	apply_dpi(GetDpiForWindow(hwnd));
 	return true;
 }
 
+void ImGuiLayer::apply_dpi(unsigned dpi)
+{
+	const float scale = dpi_scale_from(dpi);
+	ImGuiIO& io = ImGui::GetIO();
+
+	// Rebuild the font atlas at the DPI-scaled pixel size so ImGui's built-in default font is
+	// rasterized crisp at the target size, rather than bitmap-stretched the way io.FontGlobalScale
+	// would leave it. scaled_font_px() scales the one named base-size constant.
+	io.Fonts->Clear();
+	ImFontConfig cfg;
+	cfg.SizePixels = scaled_font_px(dpi);
+	io.Fonts->AddFontDefault(&cfg);
+
+	// Reset to the base dark theme, then scale spacing/padding/border sizes to match. Resetting
+	// first keeps repeated DPI changes from compounding (ScaleAllSizes multiplies the style in place).
+	ImGui::StyleColorsDark();
+	ImGui::GetStyle().ScaleAllSizes(scale);
+
+	// Drop the DX11 backend's cached font texture so it rebuilds from the new atlas next frame. Before
+	// the first frame nothing is built yet, so this is a harmless no-op during init().
+	if (initialized_)
+	{
+		ImGui_ImplDX11_InvalidateDeviceObjects();
+	}
+	dpi_scale_ = scale;
+}
+
+void ImGuiLayer::set_dpi(unsigned dpi)
+{
+	if (!initialized_ || dpi_scale_from(dpi) == dpi_scale_)
+	{
+		return; // not up yet, or the scale didn't actually change -- skip a needless atlas rebuild
+	}
+	apply_dpi(dpi);
+}
+
 void ImGuiLayer::begin_frame()
 {
 	ImGui_ImplDX11_NewFrame();

host/src/imgui_layer.hpp

@@ -22,8 +22,18 @@ public:
 	void begin_frame();
 	void end_frame();
 
+	// Rescale the overlay's font + style to a monitor DPI (dots per inch). Called from the window's
+	// deferred WM_DPICHANGED handling when the window moves to a different-DPI monitor or the display
+	// scale changes at runtime. No-op before init() or when the resulting scale is unchanged.
+	void set_dpi(unsigned dpi);
+
 private:
+	// Rebuild the font atlas at the DPI-scaled size and re-apply the scaled dark style. Used by both
+	// init() (first apply) and set_dpi() (runtime change).
+	void apply_dpi(unsigned dpi);
+
 	bool initialized_ = false;
+	float dpi_scale_ = 1.0f; // last-applied UI scale; guards set_dpi against redundant atlas rebuilds
 	// Backing storage for io.IniFilename (ImGui keeps the pointer, not a copy), so the
 	// layout .ini path must outlive the context. Empty until init() sets it.
 	std::string ini_path_;

host/src/main.cpp

@@ -387,6 +387,13 @@ int run()
 
 	while (window.pump_messages())
 	{
+		// Rescale the overlay if the window changed DPI (moved monitors, or the display scale changed).
+		// pump_messages() latches the new DPI; apply it here, outside any in-progress ImGui frame.
+		if (unsigned new_dpi = 0; window.take_dpi_change(new_dpi))
+		{
+			imgui.set_dpi(new_dpi);
+		}
+
 		// When the operator enabled "Sync flip to game frames" (Hooked source), pace the
 		// whole iteration to the game: wait for the next published frame before rendering,
 		// then present without vsync so the flip lands in lockstep with the game.
@@ -539,6 +546,11 @@ int run()
 
 int WINAPI wWinMain(HINSTANCE, HINSTANCE, LPWSTR, int)
 {
+	// Per-monitor DPI awareness (v2): make GetSystemMetrics / GetDpiForWindow report true pixels so the
+	// mirror renders at the display's native resolution instead of a virtualized one that Windows would
+	// bitmap-stretch (softening the image). We scale the ImGui overlay ourselves to stay readable. Must
+	// run before any window is created.
+	SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2);
 	// WGC requires an initialized apartment; multi-threaded suits the
 	// free-threaded frame pool.
 	winrt::init_apartment(winrt::apartment_type::multi_threaded);