A finishing activity no longer takes the next one's Surface; the library label is environment, not an argument

Hand-off from the desktop to a Steam session: the new activity attached its Surface, then the
old one's surfaceDestroyed detached it - the compositor presented nothing while gamescope sent
frames by the thousand (black screen, sound only). detach() now takes the Surface being destroyed
and ignores one that is not attached. BL_LIBRARY_LABEL was added to the guest list after the
script's mode and reached the Steam client as an argument; it is an environment variable now.

app/src/main/java/com/steamdeck/launcher/SessionActivity.kt

@@ -361,7 +361,7 @@ class SessionActivity : ComponentActivity(), SurfaceHolder.Callback {
     override fun surfaceDestroyed(holder: SurfaceHolder) {
         surfaceW = 0
         surfaceH = 0
-        CompositorHost.detach()
+        CompositorHost.detach(holder.surface)
     }
 
     /**

app/src/main/java/com/steamdeck/launcher/session/SessionService.kt

@@ -292,6 +292,8 @@ class SessionService : Service() {
         }
         // Where the guest leaves a request for another session (the desktop's Steam launchers).
         guest.add("BL_LAUNCH_DIR=" + sessionRoot.path)
+        // The second library's name, for bannerlator-steam-library; the bind itself is made below.
+        GameStorage.effective(this)?.let { guest.add("BL_LIBRARY_LABEL=" + it.label.replace('"', ' ')) }
         if (SessionState.mode == MODE_STEAM && SessionState.steamUi == "desktop") guest.add("BL_STEAM_UI=desktop")
         guest.add(LinuxRuntime.SESSION_SCRIPT)
         guest.add(SessionState.mode)
@@ -339,7 +341,6 @@ class SessionService : Service() {
             if (problem == null) {
                 File(LinuxRuntime.rootDir(this), "mnt/bannerlator-sd").mkdirs()
                 binds.add("${library.path}:/mnt/bannerlator-sd")
-                guest.add("BL_LIBRARY_LABEL=" + library.label.replace('"', ' '))
                 Log.i(TAG, "game storage: ${library.path} -> /mnt/bannerlator-sd (\"${library.label}\")")
             } else {
                 Log.w(TAG, "game storage: $problem; internal only this session")

app/src/main/java/com/steamdeck/launcher/wayland/CompositorHost.kt

@@ -34,6 +34,7 @@ object CompositorHost {
         refreshHz: Float,
     ): Boolean {
         if (started) {
+            attached = surface
             WaylandCompositor.nativeSetSurface(surface)
             resumeVsync()
             return false
@@ -43,6 +44,7 @@ object CompositorHost {
         WaylandCompositor.nativeStartWithSurface(
             surface, xdgRuntimeDir, driverPath, libraryName, nativeLibDir,
         )
+        attached = surface
         started = true
         resumeVsync()
         return true
@@ -66,6 +68,7 @@ object CompositorHost {
         WaylandCompositor.nativeSetFrameGenArmed(false, 0)
         WaylandCompositor.nativeLog("screen", "surface resized: rebinding the window, frame generation re-armed after")
         WaylandCompositor.nativeSetSurface(null)
+        attached = surface
         WaylandCompositor.nativeSetSurface(surface)
         android.os.Handler(android.os.Looper.getMainLooper()).postDelayed(rearm, 600)
     }
@@ -87,9 +90,20 @@ object CompositorHost {
      * The activity is going away. The compositor keeps running with nothing to present into —
      * the guest carries on, and its next frames land on the Surface the next activity brings.
      */
+    /** The Surface the compositor presents on; only its own owner may take it away. */
+    private var attached: Surface? = null
+
+    /**
+     * Only for the Surface that is attached. When one session hands over to the next (the
+     * desktop's Steam launchers), the new activity attaches its Surface before the old one is
+     * destroyed - and the old one's surfaceDestroyed then arrived here and took the new Surface
+     * away: gamescope's frames came in by the thousand and nothing reached the screen.
+     */
     @Synchronized
-    fun detach() {
+    fun detach(surface: Surface?) {
         if (!started) return
+        if (surface != null && attached != null && attached !== surface) return
+        attached = null
         pauseVsync()
         WaylandCompositor.nativeSetSurface(null)
     }