refactor: do not start redundant UI event transaction when one is already on Scope (#5658)

SentryGestureListener.startTracing always started a UI transaction and
only later, in applyScope, declined to bind it when the Scope already
held a manually-bound transaction. The unbound UI transaction then
gathered no children and was dropped as an idle transaction. Now we read
the Scope's bound transaction first and return early without starting a
new one when it is present.

Fixes #5491

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

Author: mvanhorn

CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Fixes
 
+- Don't start a redundant UI interaction transaction when a transaction is already bound to the Scope ([#5491](https://github.com/getsentry/sentry-java/issues/5491))
+  - Previously, `SentryGestureListener` always started a UI transaction and only afterwards skipped binding it to the Scope when a manually-bound transaction already existed, leaving the new transaction to be dropped as an idle transaction without children.
 - Fix potential NPE within `Scope.endSession()` ([#5657](https://github.com/getsentry/sentry-java/pull/5657))
 
 ### Performance

sentry-android-core/src/main/java/io/sentry/android/core/internal/gestures/SentryGestureListener.java

@@ -244,6 +244,21 @@ private void startTracing(final @NotNull UiElement target, final @NotNull Gestur
       }
     }
 
+    // if there's already a transaction bound to the Scope (e.g. started manually by the user), we
+    // skip starting a new UI transaction: it would never be bound to the Scope in applyScope, would
+    // gather no children, and would be dropped as an idle transaction without children
+    final @Nullable ITransaction[] boundTransaction = {null};
+    scopes.configureScope(scope -> boundTransaction[0] = scope.getTransaction());
+    if (boundTransaction[0] != null) {
+      options
+          .getLogger()
+          .log(
+              SentryLevel.DEBUG,
+              "Transaction won't be created for view with id: %s since there's already a transaction bound to the Scope.",
+              viewIdentifier);
+      return;
+    }
+
     // we can only bind to the scope if there's no running transaction
     final String name = getActivityName(activity) + "." + viewIdentifier;
     final String op = UI_ACTION + "." + getGestureType(eventType);

sentry-android-core/src/test/java/io/sentry/android/core/internal/gestures/SentryGestureListenerTracingTest.kt

@@ -160,6 +160,18 @@ class SentryGestureListenerTracingTest {
     sut.onSingleTapUp(fixture.event)
   }
 
+  @Test
+  fun `when a transaction is already bound to the Scope, does not start a new UI transaction`() {
+    val sut = fixture.getSut<View>()
+    val boundTransaction = SentryTracer(TransactionContext("bound", "op"), fixture.scopes)
+    whenever(fixture.scope.transaction).thenReturn(boundTransaction)
+
+    sut.onSingleTapUp(fixture.event)
+
+    verify(fixture.scopes, never()).startTransaction(any(), any<TransactionOptions>())
+    assertEquals(false, boundTransaction.isFinished)
+  }
+
   @Test
   fun `stopTracing remove transaction from scope`() {
     val sut = fixture.getSut<View>()