feat(ui): support native desktop backdrops

shared/src/commonMain/kotlin/com/vnidrop/app/App.kt

@@ -13,6 +13,7 @@ import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import androidx.lifecycle.Lifecycle
@@ -54,6 +55,8 @@ fun App(
 	dependencies: AppDependencies,
 	windowChromeTopInset: Dp = 0.dp,
 	windowContentTopStartRadius: Dp = 0.dp,
+	useNativeWindowBackdrop: Boolean = false,
+	onResolvedDarkThemeChanged: (Boolean) -> Unit = {},
 	windowChrome: (@Composable () -> Unit)? = null,
 ) {
 	val graphHolder = viewModel { AppGraphViewModel(dependencies) }
@@ -140,13 +143,19 @@ fun App(
 	}
 
 	val darkTheme = rememberResolvedDarkTheme(appState.themeMode)
+	LaunchedEffect(darkTheme, onResolvedDarkThemeChanged) {
+		onResolvedDarkThemeChanged(darkTheme)
+	}
 	PlatformSystemAppearance(darkTheme)
 	CompositionLocalProvider(LocalUiPlatform provides dependencies.environment.uiPlatform) {
 		VniDropTheme(isDarkTheme = darkTheme) {
 			Box(
 				modifier = Modifier
 					.fillMaxSize()
-					.background(LocalVniDropColors.current.backgroundSurface200),
+					.background(
+						if (useNativeWindowBackdrop) Color.Transparent
+						else LocalVniDropColors.current.backgroundSurface200,
+					),
 			) {
 				BoxWithConstraints(
 					modifier = Modifier
@@ -171,6 +180,7 @@ fun App(
 						windowClass = windowClass,
 						uiPlatform = dependencies.environment.uiPlatform,
 						mainContentTopStartRadius = windowContentTopStartRadius,
+						useNativeWindowBackdrop = useNativeWindowBackdrop,
 						onDestinationSelected = appViewModel::selectDestination,
 						overlay = {
 							VniDropSnackbarHost(graph.messages, Modifier.align(Alignment.BottomCenter))

shared/src/commonMain/kotlin/com/vnidrop/app/ui/navigation/AppNavigation.kt

@@ -55,6 +55,7 @@ fun AppSidebarNavigation(
 	style: NavigationStyle,
 	onDestinationSelected: (AppDestination) -> Unit,
 	modifier: Modifier = Modifier,
+	useNativeWindowBackdrop: Boolean = false,
 ) {
 	when (style) {
 		NavigationStyle.AndroidRail -> AndroidNavigationRail(selected, onDestinationSelected, modifier)
@@ -62,6 +63,7 @@ fun AppSidebarNavigation(
 			selected = selected,
 			onDestinationSelected = onDestinationSelected,
 			modifier = modifier,
+			useNativeWindowBackdrop = useNativeWindowBackdrop,
 		)
 		NavigationStyle.AndroidBottomBar -> error("Bottom navigation is rendered by the phone shell")
 	}
@@ -103,13 +105,14 @@ private fun DesktopSidebarNavigation(
 	selected: AppDestination,
 	onDestinationSelected: (AppDestination) -> Unit,
 	modifier: Modifier = Modifier,
+	useNativeWindowBackdrop: Boolean = false,
 ) {
 	val colors = LocalVniDropColors.current
 	Column(
 		modifier = modifier
 			.width(DesktopNavigationWidthDp.dp)
 			.fillMaxHeight()
-			.background(colors.backgroundSurface200)
+			.background(if (useNativeWindowBackdrop) Color.Transparent else colors.backgroundSurface200)
 			.padding(horizontal = 12.dp, vertical = 14.dp),
 		verticalArrangement = Arrangement.spacedBy(4.dp),
 	) {

shared/src/commonMain/kotlin/com/vnidrop/app/ui/shell/AppShell.kt

@@ -15,6 +15,7 @@ import androidx.compose.material3.Surface
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
+import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import com.vnidrop.app.UiPlatform
@@ -33,6 +34,7 @@ fun AppShell(
 	windowClass: WindowClass,
 	uiPlatform: UiPlatform,
 	mainContentTopStartRadius: Dp = 0.dp,
+	useNativeWindowBackdrop: Boolean = false,
 	onDestinationSelected: (AppDestination) -> Unit,
 	overlay: @Composable BoxScope.() -> Unit = {},
 	floatingAction: (@Composable BoxScope.() -> Unit)? = null,
@@ -40,11 +42,13 @@ fun AppShell(
 ) {
 	val colors = LocalVniDropColors.current
 	val navigationStyle = navigationStyleFor(uiPlatform, windowClass)
+	val windowSurface = if (useNativeWindowBackdrop) Color.Transparent else colors.backgroundDashCanvas
 	Surface(
 		modifier = modifier
 			.fillMaxSize()
-			.background(colors.backgroundDashCanvas),
-		color = colors.backgroundDashCanvas,
+			.background(windowSurface),
+		color = windowSurface,
+		contentColor = colors.foregroundDefault,
 	) {
 		if (navigationStyle == NavigationStyle.AndroidBottomBar) {
 			PhoneShell(
@@ -59,6 +63,7 @@ fun AppShell(
 				selectedDestination = selectedDestination,
 				navigationStyle = navigationStyle,
 				mainContentTopStartRadius = mainContentTopStartRadius,
+				useNativeWindowBackdrop = useNativeWindowBackdrop,
 				onDestinationSelected = onDestinationSelected,
 				overlay = overlay,
 				floatingAction = floatingAction,
@@ -73,6 +78,7 @@ private fun WideShell(
 	selectedDestination: AppDestination,
 	navigationStyle: NavigationStyle,
 	mainContentTopStartRadius: Dp,
+	useNativeWindowBackdrop: Boolean,
 	onDestinationSelected: (AppDestination) -> Unit,
 	overlay: @Composable BoxScope.() -> Unit,
 	floatingAction: (@Composable BoxScope.() -> Unit)?,
@@ -83,11 +89,18 @@ private fun WideShell(
 	Row(
 		modifier = Modifier
 			.fillMaxSize()
-			.then(if (roundedContent) Modifier.background(colors.backgroundSurface200) else Modifier),
+			.then(
+				if (roundedContent && !useNativeWindowBackdrop) {
+					Modifier.background(colors.backgroundSurface200)
+				} else {
+					Modifier
+				},
+			),
 	) {
 		AppSidebarNavigation(
 			selected = selectedDestination,
 			style = navigationStyle,
+			useNativeWindowBackdrop = useNativeWindowBackdrop,
 			onDestinationSelected = onDestinationSelected,
 		)
 		Box(
@@ -96,13 +109,12 @@ private fun WideShell(
 				.fillMaxSize()
 				.then(
 					if (roundedContent) {
-						Modifier
-							.clip(RoundedCornerShape(topStart = mainContentTopStartRadius))
-							.background(colors.backgroundDashCanvas)
+						Modifier.clip(RoundedCornerShape(topStart = mainContentTopStartRadius))
 					} else {
 						Modifier
 					},
-				),
+				)
+				.background(colors.backgroundDashCanvas),
 		) {
 			content()
 			floatingAction?.invoke(this)

shared/src/jvmTest/kotlin/com/vnidrop/app/ui/FoundationComposeTest.kt

@@ -4,6 +4,7 @@ import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.background
 import androidx.compose.material3.Text
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
@@ -18,7 +19,12 @@ import androidx.compose.ui.test.onNodeWithTag
 import androidx.compose.ui.test.onNodeWithText
 import androidx.compose.ui.test.onAllNodesWithText
 import androidx.compose.ui.test.performClick
+import androidx.compose.ui.test.captureToImage
+import androidx.compose.ui.test.onRoot
 import androidx.compose.ui.platform.testTag
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.toArgb
+import androidx.compose.ui.graphics.toPixelMap
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.test.v2.runComposeUiTest
 import androidx.compose.runtime.CompositionLocalProvider
@@ -55,6 +61,7 @@ import com.vnidrop.app.ui.navigation.AppDestination
 import com.vnidrop.app.ui.platform.LocalUiPlatform
 import com.vnidrop.app.ui.shell.AppShell
 import com.vnidrop.app.ui.theme.VniDropTheme
+import com.vnidrop.app.ui.theme.LocalVniDropColors
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertTrue
@@ -290,6 +297,68 @@ class FoundationComposeTest {
 		runOnIdle { assertEquals(AppDestination.Receive, selected) }
 	}
 
+	@Test
+	fun nativeWindowBackdropShowsThroughDesktopChromeButNotMainContent() = runComposeUiTest {
+		val sentinel = Color.Magenta
+		var expectedMain = Color.Unspecified
+		setContent {
+			VniDropTheme(isDarkTheme = false) {
+				expectedMain = LocalVniDropColors.current.backgroundDashCanvas
+				Box(
+					Modifier
+						.size(width = 320.dp, height = 200.dp)
+						.background(sentinel)
+						.testTag("native-backdrop-shell"),
+				) {
+					AppShell(
+						selectedDestination = AppDestination.Send,
+						windowClass = WindowClass.Desktop,
+						uiPlatform = UiPlatform.Windows,
+						mainContentTopStartRadius = 20.dp,
+						useNativeWindowBackdrop = true,
+						onDestinationSelected = {},
+					) {
+						Text("Content")
+					}
+				}
+			}
+		}
+
+		val pixels = onNodeWithTag("native-backdrop-shell").captureToImage().toPixelMap()
+		assertEquals(sentinel.toArgb(), pixels[pixels.width / 20, pixels.height * 9 / 10].toArgb())
+		assertEquals(expectedMain.toArgb(), pixels[pixels.width * 19 / 20, pixels.height * 9 / 10].toArgb())
+	}
+
+	@Test
+	fun desktopChromeKeepsSolidFallbackWithoutNativeBackdrop() = runComposeUiTest {
+		var expectedSidebar = Color.Unspecified
+		setContent {
+			VniDropTheme(isDarkTheme = false) {
+				expectedSidebar = LocalVniDropColors.current.backgroundSurface200
+				Box(
+					Modifier
+						.size(width = 320.dp, height = 200.dp)
+						.background(Color.Magenta)
+						.testTag("solid-backdrop-shell"),
+				) {
+					AppShell(
+						selectedDestination = AppDestination.Send,
+						windowClass = WindowClass.Desktop,
+						uiPlatform = UiPlatform.Windows,
+						mainContentTopStartRadius = 20.dp,
+						useNativeWindowBackdrop = false,
+						onDestinationSelected = {},
+					) {
+						Text("Content")
+					}
+				}
+			}
+		}
+
+		val pixels = onNodeWithTag("solid-backdrop-shell").captureToImage().toPixelMap()
+		assertEquals(expectedSidebar.toArgb(), pixels[pixels.width / 20, pixels.height * 9 / 10].toArgb())
+	}
+
 	@Test
 	fun androidPagesUseStaticFeatureIconsWithoutTitleDescriptions() = runComposeUiTest {
 		val actions = object : ReceiveInvitationActions {