diff --git a/clang-tools-extra/clangd/index/Background.h b/clang-tools-extra/clangd/index/Background.h
--- a/clang-tools-extra/clangd/index/Background.h
+++ b/clang-tools-extra/clangd/index/Background.h
@@ -100,7 +100,7 @@
   void boost(llvm::StringRef Tag, unsigned NewPriority);
 
   // Process items on the queue until the queue is stopped.
-  // If the queue becomes empty, OnIdle will be called (on one worker).
+  // Each time the queue becomes empty, OnIdle will be called (on one worker).
   void work(std::function<void()> OnIdle = nullptr);
 
   // Stop processing new tasks, allowing all work() calls to return soon.
diff --git a/clang-tools-extra/clangd/index/BackgroundQueue.cpp b/clang-tools-extra/clangd/index/BackgroundQueue.cpp
--- a/clang-tools-extra/clangd/index/BackgroundQueue.cpp
+++ b/clang-tools-extra/clangd/index/BackgroundQueue.cpp
@@ -46,8 +46,11 @@
     {
       std::unique_lock<std::mutex> Lock(Mu);
       ++Stat.Completed;
-      if (Stat.Active == 1 && Queue.empty()) {
-        // We just finished the last item, the queue is going idle.
+      // If we just finished the last item, the queue is going idle.
+      // Loop in case more items get scheduled and completed during OnIdle().
+      // (We treat this as going idle a second time).
+      while (Stat.Active == 1 && Queue.empty() &&
+             Stat.LastIdle != Stat.Completed) {
         assert(ShouldStop || Stat.Completed == Stat.Enqueued);
         Stat.LastIdle = Stat.Completed;
         if (OnIdle) {
diff --git a/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp b/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
--- a/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
+++ b/clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
@@ -7,6 +7,7 @@
 #include "TestTU.h"
 #include "index/Background.h"
 #include "index/BackgroundRebuild.h"
+#include "support/Threading.h"
 #include "clang/Tooling/ArgumentsAdjusters.h"
 #include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/Support/ScopedPrinter.h"
@@ -826,6 +827,41 @@
   }
 }
 
+TEST(BackgroundQueueTest, Idle) {
+  std::atomic<unsigned> TaskCount(0), IdleCount(0);
+  BackgroundQueue::Task Task([&] { ++TaskCount; });
+
+  // Set up a queue initially with 10 tasks.
+  AsyncTaskRunner ThreadPool;
+  BackgroundQueue Q;
+  for (unsigned I = 0; I < 10; ++I)
+    Q.push(Task);
+  Notification Start;
+  for (unsigned I = 0; I < 5; ++I)
+    ThreadPool.runAsync("worker", [&] {
+      Start.wait();
+      Q.work(/*OnIdle=*/[&] {
+        // Should be nothing else running now, so this assertion isn't racy.
+        EXPECT_EQ(10u + 2u * IdleCount, TaskCount);
+
+        // The first two times the queue goes idle, add a couple more tasks.
+        // This should caute OnIdle to run again.
+        if (++IdleCount <= 2) {
+          Q.push(Task);
+          Q.push(Task);
+        }
+      });
+    });
+
+  Start.notify();
+  ASSERT_TRUE(Q.blockUntilIdleForTest(/*TimeoutSeconds=*/5));
+  Q.stop();
+  ASSERT_TRUE(ThreadPool.wait(timeoutSeconds(5)));
+
+  EXPECT_EQ(3u, IdleCount)
+      << "Queue exhausts original items, then the extra two (twice)";
+}
+
 TEST(BackgroundQueueTest, Progress) {
   using testing::AnyOf;
   BackgroundQueue::Stats S;