feat(game-ui): 지도 계절 배경을 부드럽게 전환한다

계절 배경을 미리 decode하고 두 이미지 레이어로 480ms 전환한다. 계절이 없는 지도와 reduced-motion은 불필요한 애니메이션 없이 처리한다.

app/game-frontend/e2e/mainNavigation.spec.ts

@@ -15,6 +15,7 @@ const errorResponse = (path: string, message: string) => ({
 });
 const artifactRoot = process.env.MAIN_NAVIGATION_ARTIFACT_DIR;
 const autoRefreshArtifactRoot = process.env.AUTO_REFRESH_ARTIFACT_DIR;
+const mapSeasonArtifactRoot = process.env.MAP_SEASON_ARTIFACT_DIR;
 const productionBundle = process.env.PLAYWRIGHT_FRONTEND_MODE === 'production';
 const basePath = `/${(process.env.PLAYWRIGHT_GAME_BASE_PATH ?? 'che').replace(/^\/+|\/+$/g, '')}`;
 const gameProfile = process.env.PLAYWRIGHT_GAME_PROFILE ?? 'che:default';
@@ -52,6 +53,10 @@ type NavigationFixture = {
     refCommandCategories?: boolean;
     currentYear?: number;
     currentMonth?: number;
+    mapName?: string;
+    validMapImages?: boolean;
+    mapImageGate?: Promise<void>;
+    imageRequests?: string[];
     serverId?: string;
     profile?: string;
     gameIdx?: number;
@@ -143,6 +148,13 @@ const emitReadModelInvalidation = (page: Page, invalidation: ReturnType<typeof r
         );
     }, invalidation);
 
+const waitForMainRealtime = (page: Page) =>
+    expect
+        .poll(() =>
+            page.evaluate(() => (window as unknown as { __hasMainRealtime: () => boolean }).__hasMainRealtime())
+        )
+        .toBe(true);
+
 const refCommandCategoryFixture = ['개인', '내정', '군사', '인사', '계략', '국가'].map((category, index) => ({
     category,
     values: [
@@ -503,9 +515,29 @@ const installFixture = async (page: Page, state: NavigationFixture) => {
         { profile: gameProfile }
     );
 
-    await page.route('**/image/**', async (route) => {
+    page.on('request', (request) => {
-        await route.fulfill({ status: 200, contentType: 'image/jpeg', body: Buffer.from('') });
+        const url = new URL(request.url());
+        if (url.pathname.includes('/image/') || url.hostname === 'sam-image.hided.net') {
+            (state.imageRequests ??= []).push(url.pathname);
+        }
     });
+
+    const handleImageRoute = async (route: Route) => {
+        await state.mapImageGate;
+        if (state.validMapImages) {
+            const transparentPixel = Buffer.from(
+                'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M/wHwAEAQH/69aQ6wAAAABJRU5ErkJggg==',
+                'base64'
+            );
+            await route.fulfill({ status: 200, contentType: 'image/png', body: transparentPixel });
+            return;
+        }
+        await route.fulfill({ status: 200, contentType: 'image/jpeg', body: Buffer.from('') });
+    };
+    if (process.env.SAMMO_E2E_REAL_MAP_ASSETS !== '1') {
+        await page.route('**/image/**', handleImageRoute);
+        await page.route('https://sam-image.hided.net/game/**', handleImageRoute);
+    }
     await page.route('**/events**', async (route) => {
         await route.abort();
     });
@@ -657,7 +689,7 @@ const installFixture = async (page: Page, state: NavigationFixture) => {
             }
             if (operation === 'world.getMapLayout') {
                 return response({
-                    mapName: 'che',
+                    mapName: state.mapName ?? 'che',
                     cityList: [{ id: 1, name: '업', level: 8, region: 1, x: 200, y: 120, path: [] }],
                     regionMap: { 1: '하북' },
                     levelMap: { 8: '특' },
@@ -4182,6 +4214,263 @@ test('global activity, world history, and a month boundary refresh their visible
     );
 });
 
+test('seasonal map decodes the next background and crossfades it without remounting map content', async ({ page }) => {
+    const useRealAssets = process.env.SAMMO_E2E_REAL_MAP_ASSETS === '1';
+    let releaseInitialImages: () => void = () => undefined;
+    const initialImageGate = new Promise<void>((resolveGate) => {
+        releaseInitialImages = resolveGate;
+    });
+    const state: NavigationFixture = {
+        officerLevel: 5,
+        permission: 2,
+        nationLevel: 3,
+        stage: 0,
+        npcMode: 1,
+        generalMeCalls: 0,
+        operations: [],
+        currentMonth: 3,
+        validMapImages: true,
+        mapImageGate: useRealAssets ? undefined : initialImageGate,
+    };
+    await installRealtimeHarness(page);
+    await installFixture(page, state);
+    await page.setViewportSize({ width: 1200, height: 900 });
+    await waitForMain(page);
+    await waitForMainRealtime(page);
+
+    const map = page.locator('[data-main-target="map"] .map-viewer').first();
+    if (!useRealAssets) {
+        await expect(map.locator('.skeleton-line')).toHaveCount(4);
+        await expect(map.locator('.map-area')).toHaveCount(0);
+        releaseInitialImages();
+        state.mapImageGate = undefined;
+    }
+
+    const currentLayer = map.locator('[data-map-background-layer="current"]');
+    const outgoingLayer = map.locator('[data-map-background-layer="outgoing"]');
+    await expect(currentLayer.locator('img')).toHaveAttribute('src', /bg_spring\.jpg/u);
+    await expect(map.locator('.map-area')).toBeVisible();
+    await expect
+        .poll(() => state.imageRequests?.some((url) => url.endsWith('/game/map/che/bg_summer.jpg')) ?? false)
+        .toBe(true);
+
+    const initialGeometry = await map.locator('.map-area').evaluate((area) => {
+        const rect = area.getBoundingClientRect();
+        const road = area.querySelector('.map-bgroad');
+        const city = area.querySelector('.city-base');
+        Object.defineProperty(window, '__mapSeasonTransitionProbe', {
+            configurable: true,
+            value: { area, road, city },
+        });
+        return { width: rect.width, height: rect.height };
+    });
+    expect(initialGeometry).toEqual({ width: 700, height: 500 });
+
+    if (mapSeasonArtifactRoot) {
+        await mkdir(mapSeasonArtifactRoot, { recursive: true });
+        await map.screenshot({ path: resolve(mapSeasonArtifactRoot, 'season-spring-initial.png') });
+    }
+
+    state.currentMonth = 4;
+    await emitReadModelInvalidation(page, readModelInvalidation({ lobby: true, map: true }));
+    await expect(map).toContainText('185年 4月');
+    await expect(outgoingLayer).toHaveClass(/is-transitioning/u);
+    await expect(currentLayer.locator('img')).toHaveAttribute('src', /bg_summer\.jpg/u);
+    await expect(outgoingLayer.locator('img')).toHaveAttribute('src', /bg_spring\.jpg/u);
+    await page.waitForTimeout(180);
+
+    const midpointOpacity = Number.parseFloat(
+        await outgoingLayer.evaluate((element) => getComputedStyle(element).opacity)
+    );
+    expect(midpointOpacity).toBeGreaterThan(0);
+    expect(midpointOpacity).toBeLessThan(1);
+    if (mapSeasonArtifactRoot) {
+        await map.screenshot({ path: resolve(mapSeasonArtifactRoot, 'season-spring-to-summer-midpoint.png') });
+    }
+
+    await expect(outgoingLayer).not.toHaveClass(/is-transitioning/u);
+    await expect(outgoingLayer.locator('img')).toHaveCount(0);
+    const finalState = await map.locator('.map-area').evaluate((area) => {
+        const probe = (
+            window as unknown as {
+                __mapSeasonTransitionProbe: { area: Element; road: Element | null; city: Element | null };
+            }
+        ).__mapSeasonTransitionProbe;
+        const rect = area.getBoundingClientRect();
+        return {
+            areaMounted: probe.area === area,
+            roadMounted: probe.road === area.querySelector('.map-bgroad'),
+            cityMounted: probe.city === area.querySelector('.city-base'),
+            width: rect.width,
+            height: rect.height,
+        };
+    });
+    expect(finalState).toEqual({
+        areaMounted: true,
+        roadMounted: true,
+        cityMounted: true,
+        width: 700,
+        height: 500,
+    });
+    if (mapSeasonArtifactRoot) {
+        await map.screenshot({ path: resolve(mapSeasonArtifactRoot, 'season-summer-complete.png') });
+    }
+
+    await page.setViewportSize({ width: 500, height: 900 });
+    await expect
+        .poll(() =>
+            map.locator('.map-area').evaluate((area) => {
+                const rect = area.getBoundingClientRect();
+                return { width: rect.width, height: rect.height };
+            })
+        )
+        .toEqual({ width: 500, height: 357.140625 });
+
+    state.currentMonth = 7;
+    await emitReadModelInvalidation(page, readModelInvalidation({ lobby: true, map: true }));
+    await expect(map).toContainText('185年 7月');
+    await expect(outgoingLayer).toHaveClass(/is-transitioning/u);
+    await expect(currentLayer.locator('img')).toHaveAttribute('src', /bg_fall\.jpg/u);
+    await expect(outgoingLayer.locator('img')).toHaveAttribute('src', /bg_summer\.jpg/u);
+    await expect(outgoingLayer).not.toHaveClass(/is-transitioning/u);
+    await expect(outgoingLayer.locator('img')).toHaveCount(0);
+    if (mapSeasonArtifactRoot) {
+        await map.screenshot({ path: resolve(mapSeasonArtifactRoot, 'season-fall-mobile-complete.png') });
+    }
+});
+
+test('reduced-motion map swaps the decoded seasonal background without a fade', async ({ page }) => {
+    const state: NavigationFixture = {
+        officerLevel: 5,
+        permission: 2,
+        nationLevel: 3,
+        stage: 0,
+        npcMode: 1,
+        generalMeCalls: 0,
+        operations: [],
+        currentMonth: 3,
+        validMapImages: true,
+    };
+    await page.emulateMedia({ reducedMotion: 'reduce' });
+    await installRealtimeHarness(page);
+    await installFixture(page, state);
+    await page.setViewportSize({ width: 1200, height: 900 });
+    await waitForMain(page);
+    await waitForMainRealtime(page);
+
+    const map = page.locator('[data-main-target="map"] .map-viewer').first();
+    const currentLayer = map.locator('[data-map-background-layer="current"]');
+    const outgoingLayer = map.locator('[data-map-background-layer="outgoing"]');
+    await expect(currentLayer.locator('img')).toHaveAttribute('src', /bg_spring\.jpg/u);
+    await outgoingLayer.evaluate((outgoing) => {
+        let transitionCount = 0;
+        const observer = new MutationObserver(() => {
+            if (outgoing.classList.contains('is-transitioning')) transitionCount += 1;
+        });
+        observer.observe(outgoing, { attributes: true, attributeFilter: ['class'] });
+        Object.defineProperty(window, '__reducedMotionMapProbe', {
+            configurable: true,
+            value: {
+                observer,
+                get transitionCount() {
+                    return transitionCount;
+                },
+            },
+        });
+    });
+
+    state.currentMonth = 4;
+    await emitReadModelInvalidation(page, readModelInvalidation({ lobby: true, map: true }));
+    await expect(map).toContainText('185年 4月');
+    await expect(currentLayer.locator('img')).toHaveAttribute('src', /bg_summer\.jpg/u);
+    await page.waitForTimeout(100);
+    await expect(outgoingLayer).not.toHaveClass(/is-transitioning/u);
+    await expect(outgoingLayer.locator('img')).toHaveCount(0);
+    expect(
+        await outgoingLayer.evaluate(() => {
+            const probe = (
+                window as unknown as {
+                    __reducedMotionMapProbe: { observer: MutationObserver; transitionCount: number };
+                }
+            ).__reducedMotionMapProbe;
+            probe.observer.disconnect();
+            return probe.transitionCount;
+        })
+    ).toBe(0);
+});
+
+test('seasonless map keeps its fixed background and does not start a month-boundary crossfade', async ({ page }) => {
+    const state: NavigationFixture = {
+        officerLevel: 5,
+        permission: 2,
+        nationLevel: 3,
+        stage: 0,
+        npcMode: 1,
+        generalMeCalls: 0,
+        operations: [],
+        currentMonth: 3,
+        mapName: 'ludo_rathowm',
+        validMapImages: true,
+    };
+    await installRealtimeHarness(page);
+    await installFixture(page, state);
+    await page.setViewportSize({ width: 1200, height: 900 });
+    await waitForMain(page);
+    await waitForMainRealtime(page);
+
+    const map = page.locator('[data-main-target="map"] .map-viewer').first();
+    const currentLayer = map.locator('[data-map-background-layer="current"]');
+    const outgoingLayer = map.locator('[data-map-background-layer="outgoing"]');
+    await expect(currentLayer.locator('img')).toHaveAttribute('src', /map\/ludo_rathowm\/back\.jpg/u);
+    await map.locator('.map-area').evaluate((area) => {
+        const outgoing = area.querySelector('[data-map-background-layer="outgoing"]');
+        let transitionCount = 0;
+        const observer = new MutationObserver(() => {
+            if (outgoing?.classList.contains('is-transitioning')) transitionCount += 1;
+        });
+        if (outgoing) observer.observe(outgoing, { attributes: true, attributeFilter: ['class'] });
+        Object.defineProperty(window, '__seasonlessMapProbe', {
+            configurable: true,
+            value: {
+                area,
+                outgoing,
+                observer,
+                get transitionCount() {
+                    return transitionCount;
+                },
+            },
+        });
+    });
+
+    state.currentMonth = 4;
+    await emitReadModelInvalidation(page, readModelInvalidation({ lobby: true, map: true }));
+    await expect(map).toContainText('185年 4月');
+    await page.waitForTimeout(650);
+
+    await expect(currentLayer.locator('img')).toHaveAttribute('src', /map\/ludo_rathowm\/back\.jpg/u);
+    await expect(outgoingLayer).not.toHaveClass(/is-transitioning/u);
+    await expect(outgoingLayer.locator('img')).toHaveCount(0);
+    expect(state.imageRequests?.some((url) => url.includes('/game/map/che/bg_summer.jpg')) ?? false).toBe(false);
+    expect(
+        await map.locator('.map-area').evaluate((area) => {
+            const probe = (
+                window as unknown as {
+                    __seasonlessMapProbe: {
+                        area: Element;
+                        observer: MutationObserver;
+                        transitionCount: number;
+                    };
+                }
+            ).__seasonlessMapProbe;
+            probe.observer.disconnect();
+            return { areaMounted: probe.area === area, transitionCount: probe.transitionCount };
+        })
+    ).toEqual({ areaMounted: true, transitionCount: 0 });
+    if (mapSeasonArtifactRoot) {
+        await map.screenshot({ path: resolve(mapSeasonArtifactRoot, 'seasonless-ludo-complete.png') });
+    }
+});
+
 test('same-account main tabs share one realtime diff and exclude a tab while sync is off', async ({
     context,
     page,

app/game-frontend/src/components/main/MapViewer.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { computed, ref } from 'vue';
+import { computed, nextTick, onBeforeUnmount, ref, watch } from 'vue';
 import { storeToRefs } from 'pinia';
 import { useElementSize, useMediaQuery, useMouseInElement } from '@vueuse/core';
 import SkeletonLines from '../ui/SkeletonLines.vue';
@@ -7,6 +7,7 @@ import MapCityBasic from './MapCityBasic.vue';
 import MapCityDetail from './MapCityDetail.vue';
 import { useMapViewerStore } from '../../stores/mapViewer';
 import { buildAssetUrl } from '../../utils/mapAssets';
+import { resolveMapBackgroundPath, resolveMapSeason, resolveNextMapSeason } from '../../utils/mapBackground';
 import { configuredGameAssetUrl } from '../../utils/imageAssets';
 
 interface MapSummary {
@@ -90,6 +91,45 @@ const emit = defineEmits<{
 const BASE_MAP_WIDTH = 700;
 const BASE_MAP_HEIGHT = 500;
 const SMALL_MAP_SCALE = 5 / 7;
+const MAP_BACKGROUND_TRANSITION_MS = 480;
+
+const decodedImageCache = new Map<string, Promise<void>>();
+const decodedImageElements = new Map<string, HTMLImageElement>();
+
+const preloadDecodedImage = (url: string): Promise<void> => {
+    const cached = decodedImageCache.get(url);
+    if (cached) return cached;
+
+    const pending = new Promise<void>((resolve, reject) => {
+        if (typeof Image === 'undefined') {
+            resolve();
+            return;
+        }
+
+        const image = new Image();
+        image.decoding = 'async';
+        image.onload = () => {
+            if (typeof image.decode !== 'function') {
+                decodedImageElements.set(url, image);
+                resolve();
+                return;
+            }
+            void image.decode().then(() => {
+                decodedImageElements.set(url, image);
+                resolve();
+            }, reject);
+        };
+        image.onerror = () => reject(new Error(`map background image load failed: ${url}`));
+        image.src = url;
+    }).catch((error: unknown) => {
+        decodedImageCache.delete(url);
+        decodedImageElements.delete(url);
+        throw error;
+    });
+
+    decodedImageCache.set(url, pending);
+    return pending;
+};
 
 const isWide = useMediaQuery('(min-width: 1024px)');
 const mapStore = useMapViewerStore();
@@ -101,25 +141,13 @@ const {
     selectedCityId: storeSelectedCityId,
 } = storeToRefs(mapStore);
 const hasTouchInput = useMediaQuery('(any-pointer: coarse)');
+const reduceMotion = useMediaQuery('(prefers-reduced-motion: reduce)');
 
 const mapArea = ref<HTMLElement | null>(null);
 const mapBody = ref<HTMLElement | null>(null);
 const { width: mapBodyWidth } = useElementSize(mapBody);
 const { elementX, elementY } = useMouseInElement(mapArea);
 
-const resolveSeason = (month: number): string => {
-    if (month <= 3) {
-        return 'spring';
-    }
-    if (month <= 6) {
-        return 'summer';
-    }
-    if (month <= 9) {
-        return 'fall';
-    }
-    return 'winter';
-};
-
 const resolveStateClass = (state: number): CityStateClass => {
     if (state < 10) {
         return 'good';
@@ -239,7 +267,7 @@ const mapSeason = computed(() => {
     if (!props.mapData) {
         return 'spring';
     }
-    return resolveSeason(props.mapData.month);
+    return resolveMapSeason(props.mapData.month);
 });
 
 const mapTheme = computed(() => props.mapLayout?.mapName ?? 'che');
@@ -327,24 +355,14 @@ const mapSeasonClass = computed(() => {
     return `map-season-${mapSeason.value}`;
 });
 
-const mapBackgroundImage = computed(() => {
+const mapBackground = computed(() => resolveMapBackgroundPath(mapTheme.value, mapSeason.value));
-    const theme = mapTheme.value;
-    const season = mapSeason.value;
 
-    if (theme === 'ludo_rathowm') {
+const mapBackgroundImage = computed(() => resolveAsset(mapBackground.value.path));
-        return resolveAsset('map/ludo_rathowm/back.jpg');
-    }
-    if (theme === 'chess') {
-        return resolveAsset('map/chess/chessboard.png');
-    }
-    if (theme === 'pokemon_v1') {
-        return resolveAsset('map/pokemon_v1/back_pal8.png');
-    }
-    if (theme === 'cr') {
-        return resolveAsset('map/cr/bg-fs8.png');
-    }
 
-    return resolveAsset(`map/che/bg_${season}.jpg`);
+const nextSeasonBackgroundImage = computed(() => {
+    if (!mapBackground.value.seasonal) return null;
+    const nextSeason = resolveNextMapSeason(mapSeason.value);
+    return resolveAsset(resolveMapBackgroundPath(mapTheme.value, nextSeason).path);
 });
 
 const mapRoadImage = computed(() => {
@@ -361,16 +379,165 @@ const mapRoadImage = computed(() => {
     return null;
 });
 
-const mapBackgroundStyle = computed(() => ({
+const renderedBackgroundImage = ref<string | null>(null);
-    backgroundImage: mapBackgroundImage.value ? `url('${mapBackgroundImage.value}')` : 'none',
+const outgoingBackgroundImage = ref<string | null>(null);
-    backgroundSize: '100% 100%',
+const renderedBackgroundElement = ref<HTMLImageElement | null>(null);
-}));
+const backgroundReady = ref(false);
+const outgoingBackgroundVisible = ref(false);
+const backgroundTransitioning = ref(false);
 
 const mapRoadStyle = computed(() => ({
     backgroundImage: mapRoadImage.value ? `url('${mapRoadImage.value}')` : 'none',
     backgroundSize: '100% 100%',
 }));
 
+type BackgroundRequest = {
+    imageUrl: string;
+    roadUrl: string | null;
+    nextSeasonUrl: string | null;
+};
+
+const backgroundRequest = computed<BackgroundRequest | null>(() => {
+    if (!props.mapData || !props.mapLayout) return null;
+    return {
+        imageUrl: mapBackgroundImage.value,
+        roadUrl: mapRoadImage.value,
+        nextSeasonUrl: nextSeasonBackgroundImage.value,
+    };
+});
+
+const backgroundRequestKey = computed(() => {
+    const request = backgroundRequest.value;
+    return request ? `${request.imageUrl}\u0000${request.roadUrl ?? ''}` : '';
+});
+
+let pendingBackgroundRequest: BackgroundRequest | null = null;
+let backgroundWorkerRunning = false;
+let backgroundWorkerDisposed = false;
+
+const waitForPaint = () =>
+    new Promise<void>((resolve) => {
+        let settled = false;
+        const done = () => {
+            if (settled) return;
+            settled = true;
+            window.clearTimeout(timeoutId);
+            resolve();
+        };
+        const timeoutId = window.setTimeout(done, 80);
+        window.requestAnimationFrame(() => window.requestAnimationFrame(done));
+    });
+
+const waitForBackgroundTransition = () =>
+    new Promise<void>((resolve) => window.setTimeout(resolve, MAP_BACKGROUND_TRANSITION_MS));
+
+const prefetchUpcomingImages = (request: BackgroundRequest) => {
+    if (request.nextSeasonUrl) {
+        void preloadDecodedImage(request.nextSeasonUrl).catch(() => undefined);
+    }
+    const nextSeasonIcon = resolveAsset(`${resolveNextMapSeason(mapSeason.value)}.gif`);
+    void preloadDecodedImage(nextSeasonIcon).catch(() => undefined);
+};
+
+const showBackground = async (request: BackgroundRequest) => {
+    const roadPromise = request.roadUrl
+        ? preloadDecodedImage(request.roadUrl).catch(() => undefined)
+        : Promise.resolve();
+
+    try {
+        await preloadDecodedImage(request.imageUrl);
+        await roadPromise;
+    } catch {
+        if (!renderedBackgroundImage.value) {
+            // Preserve the old initial-load fallback: reveal the map and let the
+            // actual img make its normal request instead of leaving a skeleton.
+            renderedBackgroundImage.value = request.imageUrl;
+            backgroundReady.value = true;
+        }
+        return;
+    }
+
+    if (backgroundWorkerDisposed) return;
+    if (pendingBackgroundRequest && pendingBackgroundRequest.imageUrl !== request.imageUrl) return;
+
+    const currentImage = renderedBackgroundImage.value;
+    if (!currentImage) {
+        renderedBackgroundImage.value = request.imageUrl;
+        backgroundReady.value = true;
+        prefetchUpcomingImages(request);
+        return;
+    }
+    if (currentImage === request.imageUrl) {
+        backgroundReady.value = true;
+        prefetchUpcomingImages(request);
+        return;
+    }
+
+    if (reduceMotion.value) {
+        renderedBackgroundImage.value = request.imageUrl;
+        outgoingBackgroundImage.value = null;
+        outgoingBackgroundVisible.value = false;
+        backgroundTransitioning.value = false;
+        prefetchUpcomingImages(request);
+        return;
+    }
+
+    // Keep the old decoded image fully covering the new background for one
+    // paint, then fade only that outgoing layer. Road/city/control DOM remains.
+    outgoingBackgroundImage.value = currentImage;
+    outgoingBackgroundVisible.value = true;
+    backgroundTransitioning.value = false;
+    renderedBackgroundImage.value = request.imageUrl;
+    await nextTick();
+    try {
+        await renderedBackgroundElement.value?.decode();
+    } catch {
+        renderedBackgroundImage.value = currentImage;
+        outgoingBackgroundImage.value = null;
+        outgoingBackgroundVisible.value = false;
+        return;
+    }
+    await waitForPaint();
+    if (backgroundWorkerDisposed) return;
+
+    backgroundTransitioning.value = true;
+    outgoingBackgroundVisible.value = false;
+    await waitForBackgroundTransition();
+    if (backgroundWorkerDisposed) return;
+
+    outgoingBackgroundImage.value = null;
+    backgroundTransitioning.value = false;
+    prefetchUpcomingImages(request);
+};
+
+const runBackgroundWorker = async () => {
+    if (backgroundWorkerRunning) return;
+    backgroundWorkerRunning = true;
+    try {
+        while (pendingBackgroundRequest && !backgroundWorkerDisposed) {
+            const request = pendingBackgroundRequest;
+            pendingBackgroundRequest = null;
+            await showBackground(request);
+        }
+    } finally {
+        backgroundWorkerRunning = false;
+    }
+};
+
+watch(
+    backgroundRequestKey,
+    () => {
+        pendingBackgroundRequest = backgroundRequest.value;
+        void runBackgroundWorker();
+    },
+    { immediate: true }
+);
+
+onBeforeUnmount(() => {
+    backgroundWorkerDisposed = true;
+    pendingBackgroundRequest = null;
+});
+
 const detailProps = computed(() =>
     effectiveDetailMode.value
         ? {
@@ -450,7 +617,7 @@ const selectCity = (cityId: number) => {
                 </div>
             </div>
         </div>
-        <div v-if="props.loading">
+        <div v-if="props.loading || (props.mapData && props.mapLayout && !backgroundReady)">
             <SkeletonLines :lines="4" />
         </div>
         <div v-else-if="!props.mapData || !props.mapLayout" class="map-empty">지도 데이터를 불러오지 못했습니다.</div>
@@ -462,8 +629,32 @@ const selectCity = (cityId: number) => {
                 :style="{ width: mapWidth, height: mapHeight }"
                 @click="clearTouchPreview"
             >
-                <div class="map-layer map-bglayer1" :style="mapBackgroundStyle" />
+                <div class="map-layer map-bglayer1" data-map-background-layer="current">
-                <div class="map-layer map-bglayer2" />
+                    <img
+                        v-if="renderedBackgroundImage"
+                        ref="renderedBackgroundElement"
+                        class="map-background-image"
+                        :src="renderedBackgroundImage"
+                        alt=""
+                        draggable="false"
+                    />
+                </div>
+                <div
+                    class="map-layer map-bglayer2"
+                    data-map-background-layer="outgoing"
+                    :class="{
+                        'is-visible': outgoingBackgroundVisible,
+                        'is-transitioning': backgroundTransitioning,
+                    }"
+                >
+                    <img
+                        v-if="outgoingBackgroundImage"
+                        class="map-background-image"
+                        :src="outgoingBackgroundImage"
+                        alt=""
+                        draggable="false"
+                    />
+                </div>
                 <div v-if="mapRoadImage" class="map-layer map-bgroad" :style="mapRoadStyle" />
                 <component
                     :is="effectiveDetailMode ? MapCityDetail : MapCityBasic"
@@ -637,6 +828,27 @@ const selectCity = (cityId: number) => {
     pointer-events: none;
 }
 
+.map-bglayer2 {
+    opacity: 0;
+}
+
+.map-bglayer2.is-visible {
+    opacity: 1;
+}
+
+.map-bglayer2.is-transitioning {
+    transition: opacity 480ms ease-in-out;
+    will-change: opacity;
+}
+
+.map-background-image {
+    display: block;
+    width: 100%;
+    height: 100%;
+    object-fit: fill;
+    user-select: none;
+}
+
 .map-tooltip {
     position: absolute;
     z-index: 16;
@@ -698,4 +910,10 @@ const selectCity = (cityId: number) => {
 .map-empty {
     color: rgba(232, 221, 196, 0.6);
 }
+
+@media (prefers-reduced-motion: reduce) {
+    .map-bglayer2.is-transitioning {
+        transition: none;
+    }
+}
 </style>

app/game-frontend/src/utils/mapBackground.ts

@@ -0,0 +1,35 @@
+export type MapSeason = 'spring' | 'summer' | 'fall' | 'winter';
+
+const FIXED_BACKGROUND_PATHS: Readonly<Record<string, string>> = {
+    ludo_rathowm: 'map/ludo_rathowm/back.jpg',
+    chess: 'map/chess/chessboard.png',
+    pokemon_v1: 'map/pokemon_v1/back_pal8.png',
+    cr: 'map/cr/bg-fs8.png',
+};
+
+const NEXT_SEASON: Readonly<Record<MapSeason, MapSeason>> = {
+    spring: 'summer',
+    summer: 'fall',
+    fall: 'winter',
+    winter: 'spring',
+};
+
+export const resolveMapSeason = (month: number): MapSeason => {
+    if (month <= 3) return 'spring';
+    if (month <= 6) return 'summer';
+    if (month <= 9) return 'fall';
+    return 'winter';
+};
+
+export const resolveNextMapSeason = (season: MapSeason): MapSeason => NEXT_SEASON[season];
+
+export const resolveMapBackgroundPath = (theme: string, season: MapSeason): { path: string; seasonal: boolean } => {
+    const fixedPath = FIXED_BACKGROUND_PATHS[theme];
+    if (fixedPath) {
+        return { path: fixedPath, seasonal: false };
+    }
+
+    // Ref uses the CHE seasonal backgrounds for CHE, mini-CHE variants, and
+    // unknown legacy-compatible themes that do not define their own backdrop.
+    return { path: `map/che/bg_${season}.jpg`, seasonal: true };
+};

app/game-frontend/test/mapBackground.test.ts

@@ -0,0 +1,53 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+
+import { resolveMapBackgroundPath, resolveMapSeason, resolveNextMapSeason } from '../src/utils/mapBackground.ts';
+
+void describe('mapBackground', () => {
+    void it('maps month boundaries to the four Ref seasons', () => {
+        assert.equal(resolveMapSeason(1), 'spring');
+        assert.equal(resolveMapSeason(3), 'spring');
+        assert.equal(resolveMapSeason(4), 'summer');
+        assert.equal(resolveMapSeason(6), 'summer');
+        assert.equal(resolveMapSeason(7), 'fall');
+        assert.equal(resolveMapSeason(9), 'fall');
+        assert.equal(resolveMapSeason(10), 'winter');
+        assert.equal(resolveMapSeason(12), 'winter');
+    });
+
+    void it('uses seasonal CHE backgrounds for CHE and mini-CHE themes', () => {
+        assert.deepEqual(resolveMapBackgroundPath('che', 'summer'), {
+            path: 'map/che/bg_summer.jpg',
+            seasonal: true,
+        });
+        assert.deepEqual(resolveMapBackgroundPath('miniche_clean', 'winter'), {
+            path: 'map/che/bg_winter.jpg',
+            seasonal: true,
+        });
+    });
+
+    void it('keeps seasonless theme backgrounds fixed across season changes', () => {
+        for (const [theme, expectedPath] of [
+            ['ludo_rathowm', 'map/ludo_rathowm/back.jpg'],
+            ['chess', 'map/chess/chessboard.png'],
+            ['pokemon_v1', 'map/pokemon_v1/back_pal8.png'],
+            ['cr', 'map/cr/bg-fs8.png'],
+        ] as const) {
+            assert.deepEqual(resolveMapBackgroundPath(theme, 'spring'), {
+                path: expectedPath,
+                seasonal: false,
+            });
+            assert.deepEqual(resolveMapBackgroundPath(theme, 'winter'), {
+                path: expectedPath,
+                seasonal: false,
+            });
+        }
+    });
+
+    void it('cycles the next background season across a year boundary', () => {
+        assert.equal(resolveNextMapSeason('spring'), 'summer');
+        assert.equal(resolveNextMapSeason('summer'), 'fall');
+        assert.equal(resolveNextMapSeason('fall'), 'winter');
+        assert.equal(resolveNextMapSeason('winter'), 'spring');
+    });
+});