preview: keep the fullscreen close button off the car screen

With the preview filling the phone, the button sat on the car's nav bar and
swallowed taps there. It now goes only in the black beside the frame; with
no room, the back gesture leaves.

gearslip/src/main/java/app/seb3thehacker/gearslip/ui/CarPreviewScreen.kt

@@ -131,7 +131,7 @@ fun CarPreviewScreen(onBack: () -> Unit) {
     }
 
     if (fullscreen) {
-        Fullscreen(onBack) {
+        Fullscreen(width.toFloat() / height, onBack) {
             // No fillMax here: left to the bounds alone, aspectRatio takes the whole width or the
             // whole height, whichever the car's shape allows, and the rest stays black.
             frameContent(Modifier)
@@ -171,6 +171,7 @@ fun CarPreviewScreen(onBack: () -> Unit) {
  */
 @Composable
 private fun Fullscreen(
+    aspect: Float,
     onBack: () -> Unit,
     content: @Composable BoxScope.() -> Unit,
 ) {
@@ -192,19 +193,31 @@ private fun Fullscreen(
     // is on and turning it sideways/back isn't an option.
     BackHandler(onBack = onBack)
 
-    Box(
+    androidx.compose.foundation.layout.BoxWithConstraints(
         Modifier.fillMaxSize().background(Color.Black),
         contentAlignment = Alignment.Center,
     ) {
         content()
 
         // The frame swallows every touch inside it, so leaving needs either the back gesture or
-        // a button of its own. This one sits in the corner the car UI leaves emptiest.
-        FilledTonalIconButton(
-            onClick = onBack,
-            modifier = Modifier.align(Alignment.BottomStart).padding(8.dp),
-        ) {
-            Icon(Icons.Filled.Close, contentDescription = "Leave the car preview")
+        // a button of its own. The button goes in the black beside the frame, never on it: over
+        // the frame it covered the car's own buttons. A frame the screen's own shape leaves no
+        // room, and then the back gesture is the way out.
+        val frameWidth = minOf(maxWidth, maxHeight * aspect)
+        val frameHeight = minOf(maxHeight, maxWidth / aspect)
+        val room = 56.dp
+        val corner = when {
+            maxWidth - frameWidth >= room * 2 -> Alignment.BottomStart
+            maxHeight - frameHeight >= room * 2 -> Alignment.TopStart
+            else -> null
+        }
+        if (corner != null) {
+            FilledTonalIconButton(
+                onClick = onBack,
+                modifier = Modifier.align(corner).padding(8.dp),
+            ) {
+                Icon(Icons.Filled.Close, contentDescription = "Leave the car preview")
+            }
         }
     }
 }