App screens drop the back button when it would only lead to Home or the app grid, which the nav bar already does

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

@@ -83,6 +83,14 @@ class CarNavigator(startId: String) {
         stack.add(Crumb(screen, label, labelRes))
     }
 
+    /**
+     * Whether a back button on this screen leads anywhere the nav bar doesn't: false when the
+     * screen under it is Home or the app grid, which the nav bar's own buttons already reach.
+     * Android Auto shows no back button on an app's first screen for the same reason.
+     */
+    val backLeadsSomewhere: Boolean
+        get() = stack.size >= 2 && stack[stack.size - 2].screen.let { it != CarScreen.Home && it != CarScreen.Apps }
+
     /** The one way back to a clean slate: resets the whole trail to just Home. */
     fun home() {
         stack.clear()

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

@@ -259,10 +259,14 @@ fun MediaScreen(app: MediaApp, onExit: () -> Unit) {
 @Composable
 private fun AppIdentity(app: MediaApp, onExit: () -> Unit, modifier: Modifier = Modifier) {
     val icon = remember(app) { findEntry(app.component.flattenToString())?.icon }
+    val navigator = LocalCarNavigator.current
     Row(modifier, verticalAlignment = Alignment.CenterVertically) {
-        // The same back button as every other screen's header, in the same place.
+        // Shown on the same terms as every other screen's header: only when it leads somewhere
-        GsIconButton(Icons.Filled.ArrowBack, stringResource(R.string.back), onExit)
+        // the nav bar doesn't.
-        Spacer(Modifier.width(12.dp))
+        if (navigator.backLeadsSomewhere) {
+            GsIconButton(Icons.Filled.ArrowBack, stringResource(R.string.back), onExit)
+            Spacer(Modifier.width(12.dp))
+        }
         icon?.let {
             Image(it, null, Modifier.size(26.dp).clip(RoundedCornerShape(7.dp)))
             Spacer(Modifier.width(10.dp))

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

@@ -72,7 +72,7 @@ fun MessagesScreen(packageName: String) {
 
     Column(Modifier.fillMaxSize().padding(start = 16.dp, end = 16.dp, top = 16.dp), verticalArrangement = Arrangement.spacedBy(16.dp)) {
         Row(verticalAlignment = Alignment.CenterVertically, horizontalArrangement = Arrangement.spacedBy(12.dp)) {
-            GsIconButton(Icons.AutoMirrored.Filled.ArrowBack, stringResource(R.string.back), navigator::back)
+            if (navigator.backLeadsSomewhere) GsIconButton(Icons.AutoMirrored.Filled.ArrowBack, stringResource(R.string.back), navigator::back)
             entry?.icon?.let { Image(it, contentDescription = null, modifier = Modifier.size(36.dp).clip(RoundedCornerShape(8.dp))) }
             Text(label, style = MaterialTheme.typography.headlineSmall, fontWeight = FontWeight.Bold)
         }

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

@@ -19,9 +19,9 @@ import androidx.compose.ui.unit.dp
 import app.seb3thehacker.gearslip.car.theme.GsIconButton
 
 /**
- * The top of every screen reached from Home: a back button in the current theme, the title, and
+ * The top of every screen reached from Home: the title, room on the right for the screen's own
- * room on the right for the screen's own controls. A car touchscreen has no hardware back button,
+ * controls, and a back button when the screen under this one is more than Home or the app grid -
- * so this is the way out of a screen.
+ * those the nav bar already reaches, so a back button there only repeated it.
  */
 @Composable
 fun ScreenHeader(title: String, modifier: Modifier = Modifier, trailing: @Composable RowScope.() -> Unit = {}) {
@@ -31,7 +31,7 @@ fun ScreenHeader(title: String, modifier: Modifier = Modifier, trailing: @Compos
         verticalAlignment = Alignment.CenterVertically,
         horizontalArrangement = Arrangement.spacedBy(12.dp),
     ) {
-        GsIconButton(Icons.AutoMirrored.Filled.ArrowBack, stringResource(R.string.back), navigator::back)
+        if (navigator.backLeadsSomewhere) GsIconButton(Icons.AutoMirrored.Filled.ArrowBack, stringResource(R.string.back), navigator::back)
         Text(
             title,
             style = MaterialTheme.typography.headlineSmall,