fix(sidecar): absorb first-leg double-tap collision sequentially

sidecar/src/main/kotlin/dev/sanderling/sidecar/DriverBackend.kt

@@ -186,9 +186,10 @@ private fun walkForStructuralHash(node: com.fasterxml.jackson.databind.JsonNode,
 
 // overlappedDoubleTap fires the second tap while the first is still in
 // flight, so the on-device gap stays tight on transports with high per-tap
-// latency. The overlap can collide with the first tap still executing ("only
+// latency. The overlap can collide with the other tap still executing ("only
-// one gesture can be performed at a time"); the second tap then waits the
+// one gesture can be performed at a time") on either leg; the colliding leg
-// first out and lands sequentially instead of failing the step.
+// then lands sequentially after the surviving one instead of failing the
+// step.
 internal fun overlappedDoubleTap(tapAction: () -> Unit) {
     val firstTap = java.util.concurrent.CompletableFuture.runAsync { tapAction() }
     Thread.sleep(40)
@@ -199,7 +200,11 @@ internal fun overlappedDoubleTap(tapAction: () -> Unit) {
         tapAction()
         return
     }
-    firstTap.join()
+    try {
+        firstTap.join()
+    } catch (_: Throwable) {
+        tapAction()
+    }
 }
 
 data class MetricsSample(

sidecar/src/test/kotlin/dev/sanderling/sidecar/DriverServiceTest.kt

@@ -201,6 +201,24 @@ class DriverServiceTest {
         assertEquals(2, invocations.get())
     }
 
+    @Test fun overlappedDoubleTapRetriesWhenFirstLegCollides() {
+        val landed = java.util.concurrent.atomic.AtomicInteger(0)
+        val failedFirst = java.util.concurrent.atomic.AtomicBoolean(false)
+        // The async first tap loses the race and collides; the second tap
+        // succeeds. The collision must be absorbed with a sequential retry,
+        // not propagated out of the join.
+        val tapAction = {
+            if (failedFirst.compareAndSet(false, true)) {
+                Thread.sleep(60)
+                throw IllegalStateException("only one gesture can be performed at a time")
+            }
+            landed.incrementAndGet()
+            Unit
+        }
+        overlappedDoubleTap(tapAction)
+        assertEquals(2, landed.get())
+    }
+
     @Test fun doubleTapDefaultComposesTwoTaps() {
         // Interface delegation would bind the default doubleTap to the
         // delegate, bypassing the tap override, so implement the interface