perf(replay): Defer ReplayIntegration.start() off the main thread

Move the expensive work in ReplayIntegration.start() (capture strategy
creation, recorder start, root view listener registration) to the
executor service, keeping only the lightweight state checks and
lifecycle transition synchronous. This saves ~16ms on the main thread
during SentryAndroid.init() (measured on Pixel 3).

The registerRootViewListeners() call is posted to the main looper
handler since it triggers View operations via OnRootViewsChangedListener.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: runningcode

CHANGELOG.md

@@ -24,6 +24,10 @@
   ```
 - Parse ART memory and garbage collector info from ANR tombstones into ART context ([#5428](https://github.com/getsentry/sentry-java/pull/5428))
 
+### Improvements
+
+- Improve SDK init performance by deferring `ReplayIntegration.start()` off the main thread ([#XXXX](https://github.com/getsentry/sentry-java/pull/XXXX))
+
 ## 8.42.0
 
 ### Features

sentry-android-replay/src/main/java/io/sentry/android/replay/ReplayIntegration.kt

@@ -161,6 +161,7 @@ public class ReplayIntegration(
     lifecycle.currentState >= STARTED && lifecycle.currentState < STOPPED
 
   override fun start() {
+    val isFullSession: Boolean
     lifecycleLock.acquire().use {
       if (!isEnabled.get()) {
         return
@@ -174,7 +175,7 @@ public class ReplayIntegration(
         return
       }
 
-      val isFullSession = random.sample(options.sessionReplay.sessionSampleRate)
+      isFullSession = random.sample(options.sessionReplay.sessionSampleRate)
       if (!isFullSession && !options.sessionReplay.isSessionReplayForErrorsEnabled) {
         options.logger.log(
           INFO,
@@ -184,30 +185,39 @@ public class ReplayIntegration(
       }
 
       lifecycle.currentState = STARTED
-      captureStrategy =
-        replayCaptureStrategyProvider?.invoke(isFullSession)
-          ?: if (isFullSession) {
-            SessionCaptureStrategy(
-              options,
-              scopes,
-              dateProvider,
-              replayExecutor,
-              replayCacheProvider,
-            )
-          } else {
-            BufferCaptureStrategy(
-              options,
-              scopes,
-              dateProvider,
-              random,
-              replayExecutor,
-              replayCacheProvider,
-            )
-          }
-      recorder?.start()
-      captureStrategy?.start()
+    }
+
+    // Defer the expensive work (strategy creation, recorder start, listener registration)
+    // off the calling thread to avoid blocking SentryAndroid.init() on the main thread.
+    options.executorService.submitSafely(options, "ReplayIntegration.start") {
+      lifecycleLock.acquire().use {
+        captureStrategy =
+          replayCaptureStrategyProvider?.invoke(isFullSession)
+            ?: if (isFullSession) {
+              SessionCaptureStrategy(
+                options,
+                scopes,
+                dateProvider,
+                replayExecutor,
+                replayCacheProvider,
+              )
+            } else {
+              BufferCaptureStrategy(
+                options,
+                scopes,
+                dateProvider,
+                random,
+                replayExecutor,
+                replayCacheProvider,
+              )
+            }
+        recorder?.start()
+        captureStrategy?.start()
+      }
 
-      registerRootViewListeners()
+      // Post to main thread since registerRootViewListeners triggers View operations
+      // (e.g. addOnLayoutChangeListener) via the OnRootViewsChangedListener callback
+      mainLooperHandler.post { registerRootViewListeners() }
     }
   }
 

sentry-android-replay/src/test/java/io/sentry/android/replay/ReplayIntegrationWithRecorderTest.kt

@@ -18,6 +18,7 @@ import io.sentry.android.replay.ReplayIntegrationWithRecorderTest.LifecycleState
 import io.sentry.android.replay.util.ReplayShadowMediaCodec
 import io.sentry.rrweb.RRWebMetaEvent
 import io.sentry.rrweb.RRWebVideoEvent
+import io.sentry.test.ImmediateExecutorService
 import io.sentry.transport.CurrentDateProvider
 import io.sentry.transport.ICurrentDateProvider
 import io.sentry.util.thread.NoOpThreadChecker
@@ -44,7 +45,11 @@ class ReplayIntegrationWithRecorderTest {
   @get:Rule val tmpDir = TemporaryFolder()
 
   internal class Fixture {
-    val options = SentryOptions().apply { threadChecker = NoOpThreadChecker.getInstance() }
+    val options =
+      SentryOptions().apply {
+        threadChecker = NoOpThreadChecker.getInstance()
+        executorService = ImmediateExecutorService()
+      }
     val scopes = mock<IScopes>()
 
     fun getSut(

sentry-android-replay/src/test/java/io/sentry/android/replay/ReplaySmokeTest.kt

@@ -21,6 +21,7 @@ import io.sentry.SentryReplayEvent.ReplayType
 import io.sentry.android.replay.util.ReplayShadowMediaCodec
 import io.sentry.rrweb.RRWebMetaEvent
 import io.sentry.rrweb.RRWebVideoEvent
+import io.sentry.test.ImmediateExecutorService
 import io.sentry.transport.CurrentDateProvider
 import io.sentry.transport.ICurrentDateProvider
 import java.time.Duration
@@ -59,7 +60,7 @@ class ReplaySmokeTest {
   @get:Rule val tmpDir = TemporaryFolder()
 
   internal class Fixture {
-    val options = SentryOptions()
+    val options = SentryOptions().apply { executorService = ImmediateExecutorService() }
     val scope = Scope(options)
     val scopes =
       mock<IScopes> {