Keep the map loaded behind other screens

Leaving Home used to tear down the map's surface, so coming back showed black for a second or two while the nav app redrew. Home now stays composed under every other screen.

gearslip/src/main/java/app/seb3thehacker/gearslip/car/CarUi.kt

@@ -18,6 +18,7 @@ import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.foundation.background
+import androidx.compose.foundation.gestures.detectTapGestures
 import androidx.compose.ui.graphics.lerp
 import androidx.compose.ui.graphics.Brush
 import androidx.compose.foundation.layout.widthIn
@@ -176,8 +177,20 @@ fun CarUi() {
                     // Header navigation (breadcrumb trail) - off for now, kept to tweak later.
                     // BreadcrumbBar(navigator)
                     Box(Modifier.weight(1f).fillMaxWidth()) {
+                        // Home never leaves the composition; other screens cover it. Dropping it
+                        // would tear down the map's surface, and coming back would then show black
+                        // while the nav app redrew on a new one.
+                        CarHome()
+                        if (screen != CarScreen.Home) {
+                            Box(
+                                Modifier
+                                    .fillMaxSize()
+                                    .background(MaterialTheme.colorScheme.background)
+                                    // Keeps touches that miss this screen's controls off the map.
+                                    .pointerInput(Unit) { detectTapGestures { } },
+                            ) {
                                 when (screen) {
-                            CarScreen.Home -> CarHome()
+                                    CarScreen.Home -> Unit
                                     CarScreen.Apps -> CarLauncher()
                                     CarScreen.Media -> mediaApp?.let { MediaScreen(it) { navigator.back() } }
                                     CarScreen.Browse -> BrowseAppScreen(navigator)
@@ -189,6 +202,8 @@ fun CarUi() {
                                     is CarScreen.Messages -> MessagesScreen(screen.packageName)
                                     is CarScreen.App -> CarApps.find(screen.id)?.content?.invoke()
                                 }
+                            }
+                        }
                         // The dashboard is already listing them, so it needs no popup.
                         if (screen !is CarScreen.Notifications && screen != CarScreen.Dashboard) {
                             NotificationPopup(Modifier.align(Alignment.TopCenter))