[core] Scheduler: don't sleep while defer queue is non-empty (#15968)

esphome/core/scheduler.cpp

@@ -414,8 +414,27 @@ bool HOT Scheduler::cancel_retry(Component *component, uint32_t id) {
 
 optional<uint32_t> HOT Scheduler::next_schedule_in(uint32_t now) {
   // IMPORTANT: This method should only be called from the main thread (loop task).
-  // It performs cleanup and accesses items_[0] without holding a lock, which is only
+  // Accesses items_[0] and the fast-path empty checks without holding a lock, which
-  // safe when called from the main thread. Other threads must not call this method.
+  // is only safe from the main thread. Other threads must not call this method.
+  //
+  // Note: cleanup_() is only invoked on the items_[0] path below. The early returns
+  // skip it because they don't read items_[0], and Scheduler::call() at the top of
+  // every loop iteration already performs its own cleanup before the next sleep-
+  // duration computation happens.
+
+#ifndef ESPHOME_THREAD_SINGLE
+  // defer() items live in a separate queue that is drained at the top of every
+  // loop tick via process_defer_queue_(). If any are pending, the next loop
+  // iteration has work to do right now -- don't let the caller sleep.
+  if (!this->defer_empty_())
+    return 0;
+#else
+  // On single-threaded builds, defer() routes through set_timeout(..., 0) which
+  // stages in to_add_. process_to_add() runs at the top of every scheduler.call(),
+  // so anything in to_add_ becomes runnable on the next iteration; don't sleep.
+  if (!this->to_add_empty_())
+    return 0;
+#endif
 
   // If no items, return empty optional
   if (!this->cleanup_())