Ensure consistent delay behavior in immediate across platforms (#4432)

Fixes #4430

Author: dkhalanskyjb

kotlinx-coroutines-core/nativeDarwin/src/Dispatchers.kt

@@ -46,7 +46,7 @@ private class DarwinMainDispatcher(
         val timer = Timer()
         val timerBlock: TimerBlock = {
             timer.dispose()
-            continuation.resume(Unit)
+            with(continuation) { resumeUndispatched(Unit) }
         }
         timer.start(timeMillis, timerBlock)
         continuation.disposeOnCancellation(timer)

test-utils/common/src/MainDispatcherTestBase.kt

@@ -1,6 +1,7 @@
 package kotlinx.coroutines.testing
 
 import kotlinx.coroutines.*
+import kotlinx.coroutines.channels.*
 import kotlin.test.*
 
 abstract class MainDispatcherTestBase: TestBase() {
@@ -151,6 +152,26 @@ abstract class MainDispatcherTestBase: TestBase() {
         }
     }
 
+    /** Tests that [delay] runs the task inline instead of explicitly scheduling it
+     * (which can be observed by the event loop's ordering). */
+    @Test
+    @NoJs @NoWasmWasi @NoWasmJs // This test does not work for environments with a single always-on event loop
+    fun testUndispatchedAfterDelay() = runTestOrSkip {
+        launch(Dispatchers.Main.immediate) {
+            val channel = Channel<Unit>()
+            expect(1)
+            launch {
+                channel.receive()
+                expect(3)
+            }
+            delay(100)
+            checkIsMainThread()
+            expect(2)
+            channel.send(Unit)
+            finish(4)
+        }
+    }
+
     private suspend fun <R> withMainScope(block: suspend CoroutineScope.() -> R): R {
         MainScope().apply {
             return block().also { coroutineContext[Job]!!.cancelAndJoin() }