back buttons on every screen, exit to the car's own UI, smoother link

Every screen reached from Home has a themed back button in its header. A new Exit tile
asks the car to show its own interface, as Android Auto's does. All writes to the car
now go through one thread, so reading never waits on a write; LIVI no longer stalls
for two seconds at a time. Touches go to the front of the main thread's queue.

gearslip/src/main/java/app/seb3thehacker/gearslip/CarFocus.kt

@@ -0,0 +1,28 @@
+package app.seb3thehacker.gearslip
+
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+
+/**
+ * Who has the car's screen: Gearslip, or the car's own interface. Android Auto's "Exit" hands the
+ * screen back to the car with the session still running, so music keeps playing; the car's own
+ * Android Auto button brings Gearslip back. [GearslipRunner] fills in [request] while connected.
+ */
+object CarFocus {
+
+    private val projectedFlow = MutableStateFlow(true)
+
+    /** False while the car shows its own interface. */
+    val projected: StateFlow<Boolean> = projectedFlow
+
+    @Volatile internal var request: (() -> Unit)? = null
+
+    /** Asks the car to show its own interface. Does nothing when no car is connected. */
+    fun exitToCar() {
+        request?.invoke()
+    }
+
+    internal fun set(projected: Boolean) {
+        projectedFlow.value = projected
+    }
+}

gearslip/src/main/java/app/seb3thehacker/gearslip/GearslipActivity.kt

@@ -40,6 +40,8 @@ import java.io.FileOutputStream
  * rotation or theme change from recreating it and dropping the connection.
  */
 class GearslipActivity : ComponentActivity(), Projection {
+    private val mainHandler = android.os.Handler(android.os.Looper.getMainLooper())
+
 
     private var descriptor: ParcelFileDescriptor? = null
     private var runner: GearslipRunner? = null
@@ -323,7 +325,9 @@ class GearslipActivity : ComponentActivity(), Projection {
     }
 
     override fun onTouch(action: Int, actionIndex: Int, points: List<TouchPoint>) {
-        runOnUiThread {
+        // To the front of the main thread's queue: a touch waiting behind other work is lag the
+        // driver feels, while the work it jumps ahead of can wait a frame.
+        mainHandler.postAtFrontOfQueue {
             // Mirroring drives one phone touch at a time, so it follows the first finger only.
             if (PhoneMirror.active.value) points.firstOrNull()?.let { relayTouch(action, it.x, it.y) }
             else screenProjector?.dispatchTouch(action, actionIndex, points)

gearslip/src/main/java/app/seb3thehacker/gearslip/GearslipRunner.kt

@@ -41,6 +41,9 @@ class GearslipRunner(
     private val assembler = Frames.Assembler()
     private lateinit var identityTask: FutureTask<PhoneTls>
 
+    /** Sends every message, in order, so no other thread ever blocks on the USB link. See [send]. */
+    private val writer = java.util.concurrent.Executors.newSingleThreadExecutor { Thread(it, "gearslip-writer") }
+
     /** Waits for the certificate only when TLS is first needed; see [run]. */
     private val tls: PhoneTls by lazy { identityTask.get() }
 
@@ -127,6 +130,9 @@ class GearslipRunner(
 
     fun stop() {
         running = false
+        writer.shutdownNow()
+        CarFocus.request = null
+        CarFocus.set(true)
         synchronized(ackLock) { ackLock.notifyAll() } // free an encoder waiting on an ack
         videoSource?.stop()
         videoSource = null
@@ -589,6 +595,8 @@ class GearslipRunner(
                     val at = points[actionIndex]
                     log.i("<- Touch action=$action at (${at.x.toInt()},${at.y.toInt()}) pointers=${points.size}")
                 }
+                // Time the next frame against this tap: see [reportTouchLatency].
+                if (action == ACTION_DOWN || action == ACTION_UP) touchAtUs = System.nanoTime() / 1000
                 projection?.onTouch(action, actionIndex, points)
             }
 
@@ -691,14 +699,17 @@ class GearslipRunner(
             }
 
             MSG_VIDEO_FOCUS_NOTIFICATION -> {
+                // VideoFocusNotification { focus = 1; unsolicited = 2 }
                 val fields = Wire.fields(body)
-                log.i("<- VideoFocusNotification: mode=${Wire.varint(fields, 1)}")
-                // The head unit repeats this while it waits for frames; only act once.
-                if (!videoStarted) {
-                    videoStarted = true
-                    log.i("PHASE A COMPLETE - video focus granted; starting video source")
-                    startVideoSource()
-                }
+                val mode = Wire.varint(fields, 1)?.toInt() ?: VIDEO_FOCUS_PROJECTED
+                log.i("<- VideoFocusNotification: mode=$mode unsolicited=${Wire.varint(fields, 2)}")
+                onVideoFocus(mode)
+            }
+
+            // Not in aasdk's direction of travel, but LIVI sends it: log what it asks for.
+            MSG_VIDEO_FOCUS_REQUEST -> {
+                val fields = Wire.fields(body)
+                log.i("<- VideoFocusRequest from the head unit: mode=${Wire.varint(fields, 2)} reason=${Wire.varint(fields, 3)}")
             }
 
             MSG_MEDIA_ACK -> {
@@ -772,6 +783,7 @@ class GearslipRunner(
             frameRate = if (config.frameRate == 1) 60 else 30,
             mode = if (useProjection) VideoSource.Mode.SURFACE else VideoSource.Mode.TEST_CARD,
             onCodecConfig = { csd ->
+                lastCodecConfig = csd
                 send(MSG_MEDIA_CODEC_CONFIG, csd, encrypted = true, channel = videoChannelId)
                 log.i("-> CodecConfig (${csd.size} bytes SPS/PPS)")
                 log.hex("   csd", csd, limit = 64)
@@ -800,6 +812,8 @@ class GearslipRunner(
      * makes it produce fewer frames, each one whole.
      */
     private fun sendVideoFrame(data: ByteArray, presentationTimeUs: Long, keyFrame: Boolean) {
+        // The car is showing its own screen and would throw these away.
+        if (!videoProjected) return
         synchronized(ackLock) {
             val waitStart = System.currentTimeMillis()
             while (running && inFlight >= maxUnacked) {
@@ -809,6 +823,7 @@ class GearslipRunner(
                 if (since > ACK_STALL_MS) {
                     log.w("no ack for ${since}ms with $inFlight in flight - resetting the window")
                     inFlight = 0
+                    lastAckAt = System.currentTimeMillis() // one warning per stall, not one per frame
                     break
                 }
                 ackLock.wait(ACK_STALL_MS - since + 1)
@@ -828,15 +843,11 @@ class GearslipRunner(
             resyncAttempts = 0
             lastKeyframeSentAt = System.currentTimeMillis()
         }
-        try {
-            send(MSG_MEDIA_DATA, timestamp + data, encrypted = true, channel = videoChannelId)
-        } catch (e: java.io.IOException) {
-            if (running) log.w("video send failed, stopping the encoder: ${e.message}")
-            running = false
-            videoSource?.stop()
-            return
-        }
-        inFlight++
+        // Counted before it's queued: the writer may send it and the ack come back before this
+        // thread runs again, and an ack counted ahead of its frame would leave the window full.
+        synchronized(ackLock) { inFlight++ }
+        send(MSG_MEDIA_DATA, timestamp + data, encrypted = true, channel = videoChannelId)
+        reportTouchLatency(presentationTimeUs)
         framesSent++
         if (framesSent <= 3 || keyFrame && framesSent % 30 == 0) {
             log.i(
@@ -846,6 +857,74 @@ class GearslipRunner(
         }
     }
 
+    /** Whether the car is showing Gearslip; false while it shows its own interface. */
+    @Volatile private var videoProjected = true
+    /** The encoder's SPS/PPS, resent when the car hands its screen back. */
+    @Volatile private var lastCodecConfig: ByteArray? = null
+
+    /**
+     * The car says who has its screen. The first grant starts the video. After that the car may
+     * take its screen back (our Exit, or its own buttons) and later return it: while it has the
+     * screen nothing is sent, and on return the stream restarts from the codec config and a fresh
+     * keyframe, since the car's decoder may have been reset in between.
+     */
+    private fun onVideoFocus(mode: Int) {
+        val projected = mode == VIDEO_FOCUS_PROJECTED || mode == VIDEO_FOCUS_PROJECTED_NO_INPUT
+        if (!videoStarted) {
+            // The head unit repeats this while it waits for frames; only start once.
+            if (!projected) return
+            videoStarted = true
+            CarFocus.request = ::requestNativeFocus
+            log.i("PHASE A COMPLETE - video focus granted; starting video source")
+            startVideoSource()
+            return
+        }
+        if (projected == videoProjected) return
+        videoProjected = projected
+        CarFocus.set(projected)
+        if (projected) {
+            log.i("the car handed its screen back - restarting the stream")
+            // Frames sent just before the car took over may never be acked.
+            synchronized(ackLock) {
+                inFlight = 0
+                ackLock.notifyAll()
+            }
+            lastCodecConfig?.let { send(MSG_MEDIA_CODEC_CONFIG, it, encrypted = true, channel = videoChannelId) }
+            videoSource?.requestSyncFrame()
+        } else {
+            log.i("the car is showing its own screen - video paused, audio carries on")
+        }
+    }
+
+    /** Android Auto's Exit: asks the car for its own interface, leaving the session running. */
+    private fun requestNativeFocus() {
+        if (!videoStarted) return
+        // VideoFocusRequestNotification { mode = 2; reason = 3 }
+        val request = Protobuf.varintField(2, VIDEO_FOCUS_NATIVE.toLong()) +
+            Protobuf.varintField(3, VIDEO_FOCUS_REASON_LAUNCH_NATIVE.toLong())
+        send(MSG_VIDEO_FOCUS_REQUEST, request, encrypted = true, channel = videoChannelId)
+        log.i("-> VideoFocusRequest(mode=NATIVE, reason=LAUNCH_NATIVE)")
+    }
+
+    /** Arrival time of the last tap still waiting to be seen in a frame; 0 when none is. */
+    @Volatile private var touchAtUs = 0L
+
+    /**
+     * How long a tap takes to reach the car, split where the phone can see it: the UI drawing a
+     * frame after the tap, the encoder compressing it, and the send. A frame's timestamp is when
+     * the UI finished drawing it, on the same clock as [touchAtUs]. The head unit's own decode
+     * and display come on top and can't be seen from here.
+     */
+    private fun reportTouchLatency(presentationTimeUs: Long) {
+        val touchAt = touchAtUs
+        if (touchAt == 0L || presentationTimeUs <= touchAt) return
+        touchAtUs = 0L
+        val nowUs = System.nanoTime() / 1000
+        val draw = (presentationTimeUs - touchAt) / 1000
+        val encode = (nowUs - presentationTimeUs) / 1000
+        log.i("touch -> frame: ${draw + encode}ms (draw ${draw}ms, encode and wait ${encode}ms)")
+    }
+
     private fun clamp(value: Int, limit: Int?): Float = when {
         limit == null -> value.toFloat()
         value < 0 -> 0f
@@ -872,35 +951,49 @@ class GearslipRunner(
             ((messageId shr 8) and 0xFF).toByte(), (messageId and 0xFF).toByte(),
         ) + body
 
-        synchronized(output) {
-            if (payload.size <= Frames.MAX_FRAME_PAYLOAD) {
-                val framePayload = if (encrypted) tls.encrypt(payload) else payload
-                output.write(Frames.build(channel, encrypted, framePayload, messageType))
-            } else {
-                // Split exactly as aasdk does: chunk the *plaintext*, encrypt each chunk
-                // separately, and carry the total plaintext length in the FIRST frame.
-                // Video keyframes for anything busier than a test card exceed 16 KB routinely.
-                var offset = 0
-                while (offset < payload.size) {
-                    val size = minOf(Frames.MAX_FRAME_PAYLOAD, payload.size - offset)
-                    val chunk = payload.copyOfRange(offset, offset + size)
-                    val frameType = when {
-                        offset == 0 -> Frames.TYPE_FIRST
-                        offset + size >= payload.size -> Frames.TYPE_LAST
-                        else -> Frames.TYPE_MIDDLE
+        // Every write goes through one thread, in order. The reading thread must never wait on
+        // a write: a head unit like LIVI writes its ack before it reads on, so a reader stuck
+        // behind a big video frame (to answer a ping) and a head unit stuck writing that ack
+        // held each other up until something timed out, about two seconds a time.
+        try {
+            writer.execute {
+                try {
+                    if (payload.size <= Frames.MAX_FRAME_PAYLOAD) {
+                        val framePayload = if (encrypted) tls.encrypt(payload) else payload
+                        output.write(Frames.build(channel, encrypted, framePayload, messageType))
+                    } else {
+                        // Split exactly as aasdk does: chunk the *plaintext*, encrypt each chunk
+                        // separately, and carry the total plaintext length in the FIRST frame.
+                        // Video keyframes for anything busier than a test card exceed 16 KB routinely.
+                        var offset = 0
+                        while (offset < payload.size) {
+                            val size = minOf(Frames.MAX_FRAME_PAYLOAD, payload.size - offset)
+                            val chunk = payload.copyOfRange(offset, offset + size)
+                            val frameType = when {
+                                offset == 0 -> Frames.TYPE_FIRST
+                                offset + size >= payload.size -> Frames.TYPE_LAST
+                                else -> Frames.TYPE_MIDDLE
+                            }
+                            val framePayload = if (encrypted) tls.encrypt(chunk) else chunk
+                            output.write(
+                                Frames.build(
+                                    channel, encrypted, framePayload, messageType, frameType,
+                                    if (frameType == Frames.TYPE_FIRST) payload.size else null,
+                                ),
+                            )
+                            offset += size
+                        }
+                        splitMessages++
                     }
-                    val framePayload = if (encrypted) tls.encrypt(chunk) else chunk
-                    output.write(
-                        Frames.build(
-                            channel, encrypted, framePayload, messageType, frameType,
-                            if (frameType == Frames.TYPE_FIRST) payload.size else null,
-                        ),
-                    )
-                    offset += size
+                    output.flush()
+                } catch (e: java.io.IOException) {
+                    if (running) log.w("send failed, stopping: ${e.message}")
+                    running = false
+                    videoSource?.stop()
                 }
-                splitMessages++
             }
-            output.flush()
+        } catch (e: java.util.concurrent.RejectedExecutionException) {
+            // The session is over; nothing more goes out.
         }
     }
 
@@ -1039,6 +1132,9 @@ class GearslipRunner(
         const val STREAM_MEDIA = 3
         const val CODEC_H264_BP = 3
         const val VIDEO_FOCUS_PROJECTED = 1
+        const val VIDEO_FOCUS_NATIVE = 2
+        const val VIDEO_FOCUS_PROJECTED_NO_INPUT = 4
+        const val VIDEO_FOCUS_REASON_LAUNCH_NATIVE = 2
         const val SESSION_ID = 1
         const val MSG_INPUT_REPORT = 32769
         const val MSG_KEY_BINDING_REQUEST = 32770

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

@@ -60,7 +60,7 @@ fun CarDashboardScreen() {
             .padding(horizontal = 24.dp, vertical = 16.dp),
         verticalArrangement = Arrangement.spacedBy(24.dp),
     ) {
-        Text("Today", style = MaterialTheme.typography.headlineMedium, fontWeight = FontWeight.Bold)
+        ScreenHeader("Today")
         WeatherCard(context)
         AgendaCard(context)
         NotificationsSection()

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

@@ -167,7 +167,7 @@ internal fun rememberAllEntries(): List<Entry> {
 }
 
 /** The launcher's groups, in the order they're laid out. */
-private enum class AppGroup { PHONE, MAPS, MUSIC, MESSAGING, WEB, SCREEN_SHARING, OTHER, SETTINGS }
+private enum class AppGroup { PHONE, MAPS, MUSIC, MESSAGING, WEB, SCREEN_SHARING, OTHER, SETTINGS, EXIT }
 
 private fun groupOf(entry: Entry, mediaPackages: Set<String>): AppGroup {
     entry.builtIn?.let {
@@ -175,6 +175,7 @@ private fun groupOf(entry: Entry, mediaPackages: Set<String>): AppGroup {
             BuiltInApps.Phone -> AppGroup.PHONE
             BuiltInApps.Web -> AppGroup.WEB
             BuiltInApps.ScreenSharing -> AppGroup.SCREEN_SHARING
+            BuiltInApps.Exit -> AppGroup.EXIT
             else -> AppGroup.SETTINGS
         }
     }

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

@@ -74,7 +74,7 @@ fun CarSettingsScreen() {
             .padding(horizontal = 24.dp, vertical = 16.dp),
         verticalArrangement = Arrangement.spacedBy(20.dp),
     ) {
-        Text("Settings", style = MaterialTheme.typography.headlineMedium, fontWeight = FontWeight.Bold)
+        ScreenHeader("Settings")
 
         ChoiceRow(
             "Open on connect",

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

@@ -40,11 +40,26 @@ import app.seb3thehacker.gearslip.car.theme.GsButton
 import app.seb3thehacker.gearslip.car.theme.GsTone
 import app.seb3thehacker.gearslip.host.CarAppConnection
 import app.seb3thehacker.gearslip.media.CarMedia
+import app.seb3thehacker.gearslip.CarFocus
+import androidx.compose.ui.graphics.SolidColor
+import androidx.compose.ui.graphics.vector.PathParser
 
 /** What a built-in app's id carries in a pin, so it can't collide with an installed app's component. */
 internal const val BUILT_IN_PREFIX = "gearslip:"
 
 /** One of Gearslip's own apps: a screen, with nothing running behind it once you leave. */
+/** Material's "directions car", drawn here because only the core icon set is bundled. */
+private val CarGlyph: ImageVector = ImageVector.Builder("Car", 24.dp, 24.dp, 24f, 24f).addPath(
+    PathParser().parsePathString(
+        "M18.92,6.01C18.72,5.42 18.16,5 17.5,5h-11c-0.66,0 -1.21,0.42 -1.42,1.01L3,12v8c0,0.55 0.45,1 1,1h1" +
+            "c0.55,0 1,-0.45 1,-1v-1h12v1c0,0.55 0.45,1 1,1h1c0.55,0 1,-0.45 1,-1v-8l-2.08,-5.99z" +
+            "M6.5,16c-0.83,0 -1.5,-0.67 -1.5,-1.5S5.67,13 6.5,13s1.5,0.67 1.5,1.5S7.33,16 6.5,16z" +
+            "M17.5,16c-0.83,0 -1.5,-0.67 -1.5,-1.5s0.67,-1.5 1.5,-1.5 1.5,0.67 1.5,1.5 -0.67,1.5 -1.5,1.5z" +
+            "M5,11l1.5,-4.5h11L19,11L5,11z",
+    ).toNodes(),
+    fill = SolidColor(Color.Black),
+).build()
+
 internal class BuiltInApp(
     val id: String,
     val label: String,
@@ -60,7 +75,10 @@ internal object BuiltInApps {
     val VehicleData = BuiltInApp("vehicle", "Vehicle data", Icons.Filled.Info, CarScreen.VehicleData) { it.vehicleData() }
     val Settings = BuiltInApp("settings", "Settings", Icons.Filled.Settings, CarScreen.Settings) { it.settings() }
 
-    val all = listOf(Web, ScreenSharing, Phone, VehicleData, Settings)
+    /** Hands the screen back to the car's own interface, like Android Auto's Exit. */
+    val Exit = BuiltInApp("exit", "Exit", CarGlyph, CarScreen.Home) { CarFocus.exitToCar() }
+
+    val all = listOf(Web, ScreenSharing, Phone, VehicleData, Settings, Exit)
     val entries: List<Entry> = all.map { Entry(it.label, icon = null, builtIn = it) }
 }
 

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

@@ -53,6 +53,8 @@ fun PhoneAppsScreen() {
     }
 
     Column(Modifier.fillMaxSize().padding(horizontal = 20.dp, vertical = 12.dp)) {
+        ScreenHeader("Screen sharing")
+        Spacer(Modifier.height(12.dp))
         if (!TouchRelayService.enabled) {
             TouchRelayWarning()
             Spacer(Modifier.height(12.dp))

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

@@ -101,13 +101,7 @@ fun PhoneDialerScreen() {
 
     Column(Modifier.fillMaxSize().padding(16.dp), verticalArrangement = Arrangement.spacedBy(16.dp)) {
         // The tabs share the title's row, so the list and keypad get the height back.
-        Row(Modifier.fillMaxWidth(), verticalAlignment = Alignment.CenterVertically) {
-            Text(
-                "Phone",
-                style = MaterialTheme.typography.headlineSmall,
-                fontWeight = FontWeight.Bold,
-                modifier = Modifier.weight(1f),
-            )
+        ScreenHeader("Phone") {
             if (hasPermissions) {
                 Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
                     DialerTab("Contacts", tab == TAB_CONTACTS, Modifier.width(140.dp)) { tab = TAB_CONTACTS }

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

@@ -0,0 +1,43 @@
+package app.seb3thehacker.gearslip.car
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.RowScope
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextOverflow
+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
+ * room on the right for the screen's own controls. A car touchscreen has no hardware back button,
+ * so this is the way out of a screen.
+ */
+@Composable
+fun ScreenHeader(title: String, modifier: Modifier = Modifier, trailing: @Composable RowScope.() -> Unit = {}) {
+    val navigator = LocalCarNavigator.current
+    Row(
+        modifier.fillMaxWidth(),
+        verticalAlignment = Alignment.CenterVertically,
+        horizontalArrangement = Arrangement.spacedBy(12.dp),
+    ) {
+        GsIconButton(Icons.AutoMirrored.Filled.ArrowBack, "Back", navigator::back)
+        Text(
+            title,
+            style = MaterialTheme.typography.headlineSmall,
+            fontWeight = FontWeight.Bold,
+            maxLines = 1,
+            overflow = TextOverflow.Ellipsis,
+            modifier = Modifier.weight(1f),
+        )
+        trailing()
+    }
+}

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

@@ -28,7 +28,7 @@ fun VehicleDataScreen() {
     val readings by CarSensors.readings.collectAsState()
 
     Column(Modifier.fillMaxSize().padding(horizontal = 24.dp, vertical = 16.dp)) {
-        Text("Vehicle data", style = MaterialTheme.typography.headlineMedium, fontWeight = FontWeight.Bold)
+        ScreenHeader("Vehicle data")
         Text(
             "Raw sensor channel from the connected head unit. Field meaning is a best guess - " +
                 "watch a row change while you do the thing (brake, drive, headlights) to confirm it.",

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

@@ -41,6 +41,7 @@ fun WeatherScreen() {
         Modifier.fillMaxSize().verticalScroll(rememberScrollState()).padding(horizontal = 24.dp, vertical = 16.dp),
         verticalArrangement = Arrangement.spacedBy(20.dp),
     ) {
+        ScreenHeader("Weather")
         when (val s = state) {
             WeatherState.Loading -> Text("Getting the weather…", style = MaterialTheme.typography.titleMedium)
             is WeatherState.Unavailable -> Text(

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

@@ -10,6 +10,8 @@ import android.webkit.WebView
 import android.webkit.WebViewClient
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.ui.unit.dp
+import androidx.compose.foundation.layout.padding
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.getValue
@@ -53,6 +55,8 @@ fun WebApp(content: String) {
     }
 
     Column(Modifier.fillMaxSize()) {
+        // Back steps through the page's history first (the interceptor above), then leaves.
+        ScreenHeader("Web", Modifier.padding(horizontal = 16.dp, vertical = 8.dp))
         AndroidView(
             modifier = Modifier.weight(1f).fillMaxSize(),
             factory = { context ->