protocol: open the media channel when a car holds back its screen

Older head units (2017 Pilot, 2020 Civic, 2024 Mirage) never show our
screen while the media channel is closed, which it is when music goes
over Bluetooth. If the screen hasn't come 5 seconds after asking, open
the channel and ask again.

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

@@ -1006,13 +1006,8 @@ class GearslipRunner(
                     "${videoConfigs.getOrNull(selectedConfigIndex ?: -1)?.resolutionName ?: "?"} " +
                         "(head unit offered ${offered.size}), max_unacked=$maxUnacked",
                 )
-                // VideoFocusRequestNotification { mode = 2; reason = 3 }. Reason 1 means "the phone's
-                // screen is off", which a 2020 Civic may have taken as a reason to keep its own
-                // screen (issue #43); 0 is UNKNOWN.
-                val focus = Protobuf.varintField(2, VIDEO_FOCUS_PROJECTED.toLong()) +
-                    Protobuf.varintField(3, 0L)
-                send(MSG_VIDEO_FOCUS_REQUEST, focus, encrypted = true, channel = videoChannelId)
-                log.i("-> VideoFocusRequest(mode=PROJECTED)")
+                requestProjectedFocus()
+                if (CarAudio.openSink != null) watchForWithheldScreen()
             }
 
             MSG_VIDEO_FOCUS_NOTIFICATION -> {
@@ -1288,6 +1283,38 @@ class GearslipRunner(
         }
     }
 
+    /** Asks the car for its screen; the answer comes to [onVideoFocus]. */
+    private fun requestProjectedFocus() {
+        // VideoFocusRequestNotification { mode = 2; reason = 3 }. Reason 1 means "the phone's
+        // screen is off", which a 2020 Civic may have taken as a reason to keep its own
+        // screen (issue #43); 0 is UNKNOWN.
+        val focus = Protobuf.varintField(2, VIDEO_FOCUS_PROJECTED.toLong()) +
+            Protobuf.varintField(3, 0L)
+        send(MSG_VIDEO_FOCUS_REQUEST, focus, encrypted = true, channel = videoChannelId)
+        log.i("-> VideoFocusRequest(mode=PROJECTED)")
+    }
+
+    /**
+     * Some older head units keep their own screen until every channel the phone was offered is
+     * open: a 2017 Pilot, a 2020 Civic and a 2024 Mirage (issues #43, #49, #57) never showed
+     * ours with the media channel closed, and the Mirage worked on 0.3.01, which opened it. With
+     * music over Bluetooth that channel waits for the car to switch source, so if the screen
+     * hasn't come a few seconds after asking, open it and ask again. Cars that hand the screen
+     * straight over keep their own source as before.
+     */
+    private fun watchForWithheldScreen() {
+        Thread({
+            Thread.sleep(WITHHELD_SCREEN_MS)
+            if (!running || videoStarted) return@Thread
+            val open = CarAudio.openSink ?: return@Thread
+            log.w("no screen from the car after ${WITHHELD_SCREEN_MS / 1000}s - opening the media channel and asking again")
+            open()
+            // Give the car the channel's open response before it hears the request again.
+            Thread.sleep(1_000)
+            if (running && !videoStarted) requestProjectedFocus()
+        }, "gearslip-focus-fallback").apply { isDaemon = true }.start()
+    }
+
     /** Android Auto's Exit: asks the car for its own interface, leaving the session running. */
     private fun requestNativeFocus() {
         if (!videoStarted) return
@@ -1569,6 +1596,7 @@ class GearslipRunner(
         const val ACTION_UP = 1
 
         const val ACK_STALL_MS = 2_000L
+        const val WITHHELD_SCREEN_MS = 5_000L
         const val AUDIO_TRACE_MESSAGES = 6
         const val GUIDANCE_SESSION_ID = 1
         const val PERIODIC_KEYFRAME_MS = 5_000L