fix(sidecar): land the second double-tap sequentially on gesture collision

The overlapped second tap can hit the XCTest runner while the first
gesture is still executing ('only one gesture can be performed at a
time'), failing the step. The second tap now waits the first out and
retries once, keeping the tight gap on the happy path.

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

@@ -179,6 +179,24 @@ private fun walkForStructuralHash(node: com.fasterxml.jackson.databind.JsonNode,
     out.append(')')
 }
 
+// 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
+// one gesture can be performed at a time"); the second tap then waits the
+// first out and lands sequentially instead of failing the step.
+internal fun overlappedDoubleTap(tapAction: () -> Unit) {
+    val firstTap = java.util.concurrent.CompletableFuture.runAsync { tapAction() }
+    Thread.sleep(40)
+    try {
+        tapAction()
+    } catch (_: Throwable) {
+        runCatching { firstTap.join() }
+        tapAction()
+        return
+    }
+    firstTap.join()
+}
+
 data class MetricsSample(
     val cpuPercent: Double,
     val heapBytes: Long,
@@ -760,12 +778,7 @@ class IosDriverBackend(private val udid: String) : DriverBackend {
     // turnaround instead of a full transport round trip. Sequential requests
     // leave a gap wide enough for the app to navigate between the taps.
     override fun doubleTap(x: Int, y: Int): Unit = withReconnect(replay = false) {
-        val point = maestro.Point(x, y)
-        val firstTap = java.util.concurrent.CompletableFuture.runAsync { driver.tap(point) }
-        Thread.sleep(40)
-        driver.tap(point)
-        firstTap.join()
-        Unit
+        overlappedDoubleTap { driver.tap(maestro.Point(x, y)) }
     }
 
     override fun longPress(x: Int, y: Int) = withReconnect(replay = false) { driver.longPress(maestro.Point(x, y)) }

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

@@ -102,6 +102,47 @@ class DriverServiceTest {
         assertEquals(io.grpc.Status.Code.UNAVAILABLE, thrown.status.code)
     }
 
+    // The vendored iOS client throws failures that do not extend Exception;
+    // they must still map to a status error instead of killing the RPC as a
+    // channel-level Unknown the runner cannot classify.
+    @Test fun nonExceptionThrowableMapsToInternal() {
+        val backend = object : DriverBackend by StubDriverBackend("android") {
+            override fun inputText(text: String) {
+                throw Throwable("only one gesture can be performed at a time")
+            }
+        }
+        val client = newClient(backend)
+
+        val thrown = kotlin.test.assertFailsWith<io.grpc.StatusRuntimeException> {
+            client.inputText(Text.newBuilder().setValue("hello").build())
+        }
+        assertEquals(io.grpc.Status.Code.INTERNAL, thrown.status.code)
+        assertTrue(thrown.status.description.orEmpty().contains("only one gesture"))
+    }
+
+    @Test fun overlappedDoubleTapLandsTwoTaps() {
+        val invocations = java.util.concurrent.atomic.AtomicInteger(0)
+        overlappedDoubleTap { invocations.incrementAndGet() }
+        assertEquals(2, invocations.get())
+    }
+
+    @Test fun overlappedDoubleTapRetriesSequentiallyOnGestureCollision() {
+        val invocations = java.util.concurrent.atomic.AtomicInteger(0)
+        val inFlight = java.util.concurrent.atomic.AtomicBoolean(false)
+        // Mimic the XCTest runner: a tap issued while another gesture is
+        // still executing fails instead of queuing.
+        val tapAction = {
+            if (!inFlight.compareAndSet(false, true)) {
+                throw IllegalStateException("only one gesture can be performed at a time")
+            }
+            invocations.incrementAndGet()
+            Thread.sleep(150)
+            inFlight.set(false)
+        }
+        overlappedDoubleTap(tapAction)
+        assertEquals(2, invocations.get())
+    }
+
     @Test fun doubleTapDefaultComposesTwoTaps() {
         // Interface delegation would bind the default doubleTap to the
         // delegate, bypassing the tap override, so implement the interface