Dynamically size the fullscreen window; present design space via LETTERBOX

SDL_CreateWindow hardcoded a 1920x1080 backbuffer while SDL_WINDOW_FULLSCREEN
let compositors run a different output mode; the mismatched buffer reached
the panel unscaled — quarter-size in the top-left on 4K sets, cropped ("4x
too large") on smaller non-1080p panels under cage/Wayland. Logical
presentation never got to map the fixed design space because its target
(the backbuffer) was pinned to stale dimensions.

Size the fullscreen window from SDL_GetCurrentDisplayMode so the first
commit already matches scanout 1:1, switch logical presentation to LETTERBOX
(aspect-safe bars on non-16:9 panels), and route both through new
kDesignWidth/kDesignHeight constants. Harden getLayoutSize() to always write
its out params and null-check the renderer/render-window before use.

Log requested/window/pixel-buffer/render-output sizes at startup and on
every resize event so field units self-diagnose this bug class into
~/.local/radiantwave/log.txt. SDL_WINDOW_HIGH_PIXEL_DENSITY stays off on
purpose: kiosk panels are not HiDPI, and keeping the pixel buffer equal to
the display mode keeps compositor scanout unscaled.

src/logical_size.h

@@ -4,14 +4,48 @@
 
 namespace radiantwave {
 
-inline void getLayoutSize(SDL_Renderer* renderer, int* w, int* h)
+// Design resolution every page lays out against. The renderer's logical
+// presentation (LETTERBOX) scales this fixed space onto whatever
+// window/backbuffer the display actually provides.
+inline constexpr int kDesignWidth = 1920;
+inline constexpr int kDesignHeight = 1080;
+
+// Returns the size pages should lay out against: the renderer's logical size
+// when presentation is active, otherwise the real window size. The output
+// params are always written — design constants are the last-resort fallback.
+inline void getLayoutSize(SDL_Renderer *renderer, int *w, int *h)
 {
-    SDL_RendererLogicalPresentation mode = SDL_LOGICAL_PRESENTATION_DISABLED;
-    if (!SDL_GetRenderLogicalPresentation(renderer, w, h, &mode) ||
-        mode == SDL_LOGICAL_PRESENTATION_DISABLED || *w <= 0 || *h <= 0)
-    {
-        SDL_GetWindowSize(SDL_GetRenderWindow(renderer), w, h);
-    }
+	if (!w || !h)
+		return;
+	*w = kDesignWidth;
+	*h = kDesignHeight;
+
+	if (!renderer)
+		return;
+
+	SDL_RendererLogicalPresentation mode = SDL_LOGICAL_PRESENTATION_DISABLED;
+	int logicalW = 0;
+	int logicalH = 0;
+	if (SDL_GetRenderLogicalPresentation(renderer, &logicalW, &logicalH,
+										 &mode) &&
+		mode != SDL_LOGICAL_PRESENTATION_DISABLED && logicalW > 0 &&
+		logicalH > 0) {
+		*w = logicalW;
+		*h = logicalH;
+		return;
+	}
+
+	SDL_Window *window = SDL_GetRenderWindow(renderer);
+	if (!window)
+		return;
+
+	int windowW = 0;
+	int windowH = 0;
+	SDL_GetWindowSize(window, &windowW, &windowH);
+	if (windowW > 0 && windowH > 0) {
+		*w = windowW;
+		*h = windowH;
+	}
 }
 
 } // namespace radiantwave

src/main.cpp

@@ -4,6 +4,7 @@
 
 #include "app_state.h"
 #include "home.h"
+#include "logical_size.h"
 #include "logger/logger.h"
 #include "provisioning.h"
 
@@ -32,11 +33,26 @@ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[]) {
 
 	auto *state = new AppState{};
 
-	// SDL_WINDOW_FULLSCREEN fills the desktop's native resolution, so the
-	// compositor never has to scale the moving scroller (scaling moving text
-	// causes flicker).
-	state->window =
-		SDL_CreateWindow("RadiantWave", 1920, 1080, SDL_WINDOW_FULLSCREEN);
+	// Size the fullscreen window from the display's actual mode so the very
+	// first buffer commit already matches what the compositor scanouts 1:1.
+	// Requesting a hardcoded size left the backbuffer at that size on
+	// displays running a different mode, and frames reached the panel
+	// unscaled (quarter-size on 4K, cropped on smaller panels).
+	const SDL_DisplayMode *displayMode =
+		SDL_GetCurrentDisplayMode(SDL_GetPrimaryDisplay());
+	int windowWidth =
+		(displayMode && displayMode->w > 0) ? displayMode->w
+											: radiantwave::kDesignWidth;
+	int windowHeight =
+		(displayMode && displayMode->h > 0) ? displayMode->h
+											: radiantwave::kDesignHeight;
+
+	// Borderless desktop fullscreen: the compositor never has to scale the
+	// moving scroller (scaling moving text causes flicker). Deliberately no
+	// SDL_WINDOW_HIGH_PIXEL_DENSITY — kiosk panels are not HiDPI, so keeping
+	// the pixel buffer equal to the display mode keeps scanout 1:1.
+	state->window = SDL_CreateWindow("RadiantWave", windowWidth, windowHeight,
+									 SDL_WINDOW_FULLSCREEN);
 	if (!state->window) {
 		SDL_Log("Failed to create window: %s", SDL_GetError());
 		delete state;
@@ -59,16 +75,38 @@ SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[]) {
 							std::string(SDL_GetError()) + ")");
 	}
 
-	// Lock the renderer to a 1920x1080 logical coordinate space.  All page
-	// layout code already targets this resolution, so everything renders at
-	// the correct size on any display — the GPU scales to native pixels.
-	if (!SDL_SetRenderLogicalPresentation(state->renderer, 1920, 1080,
-										  SDL_LOGICAL_PRESENTATION_STRETCH)) {
+	// All page layout targets the fixed design resolution; LETTERBOX maps it
+	// onto the real window with aspect preserved (bars on non-16:9 panels).
+	if (!SDL_SetRenderLogicalPresentation(state->renderer,
+										  radiantwave::kDesignWidth,
+										  radiantwave::kDesignHeight,
+										  SDL_LOGICAL_PRESENTATION_LETTERBOX)) {
 		radiantwave::logger::Logger::instance().warning(
 			"Graphics", "Could not set logical render size; layout may be "
 						"wrong on HiDPI displays");
 	}
 
+	{
+		// Record how the requested mode, window, pixel buffer, and render
+		// output actually compare — this is the evidence trail for
+		// display-scaling problems on field units.
+		int windowW = 0, windowH = 0;
+		SDL_GetWindowSize(state->window, &windowW, &windowH);
+		int pixelW = 0, pixelH = 0;
+		SDL_GetWindowSizeInPixels(state->window, &pixelW, &pixelH);
+		int outputW = 0, outputH = 0;
+		SDL_GetRenderOutputSize(state->renderer, &outputW, &outputH);
+		radiantwave::logger::Logger::instance().info(
+			"Graphics",
+			"Requested " + std::to_string(windowWidth) + "x" +
+				std::to_string(windowHeight) + ", window " +
+				std::to_string(windowW) + "x" + std::to_string(windowH) +
+				", pixels " + std::to_string(pixelW) + "x" +
+				std::to_string(pixelH) + ", render output " +
+				std::to_string(outputW) + "x" + std::to_string(outputH) +
+				", presentation LETTERBOX");
+	}
+
 	SDL_HideCursor();
 
 	// Initialize configuration, database, and network manager
@@ -157,6 +195,19 @@ SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event) {
 		state.quitRequested = true;
 	}
 
+	if (event->type == SDL_EVENT_WINDOW_RESIZED) {
+		radiantwave::logger::Logger::instance().info(
+			"Graphics", "Window resized to " +
+							std::to_string(event->window.data1) + "x" +
+							std::to_string(event->window.data2));
+		// Logical presentation persists across resizes; re-apply defensively
+		// in case backend state was reset underneath us.
+		SDL_SetRenderLogicalPresentation(state.renderer,
+										 radiantwave::kDesignWidth,
+										 radiantwave::kDesignHeight,
+										 SDL_LOGICAL_PRESENTATION_LETTERBOX);
+	}
+
 	if (!state.keybinds.handleEvent(*event)) {
 		state.pages.handleEvent(*event);
 	}