diff --git a/clang-tools-extra/clangd/TUScheduler.cpp b/clang-tools-extra/clangd/TUScheduler.cpp
--- a/clang-tools-extra/clangd/TUScheduler.cpp
+++ b/clang-tools-extra/clangd/TUScheduler.cpp
@@ -276,6 +276,7 @@
   /// Set to true to signal run() to finish processing.
   bool Done;                    /* GUARDED_BY(Mutex) */
   std::deque<Request> Requests; /* GUARDED_BY(Mutex) */
+  bool RunningTask = false;     /* GUARDED_BY(Mutex) */
   mutable std::condition_variable RequestsCV;
   /// Guards the callback that publishes results of AST-related computations
   /// (diagnostics, highlightings) and file statuses.
@@ -370,7 +371,8 @@
 #ifndef NDEBUG
   std::lock_guard<std::mutex> Lock(Mutex);
   assert(Done && "handle was not destroyed");
-  assert(Requests.empty() && "unprocessed requests when destroying ASTWorker");
+  assert(Requests.empty() && !RunningTask &&
+         "unprocessed requests when destroying ASTWorker");
 #endif
 }
 
@@ -745,7 +747,8 @@
         wait(Lock, RequestsCV, Wait);
       }
       Req = std::move(Requests.front());
-      // Leave it on the queue for now, so waiters don't see an empty queue.
+      Requests.pop_front();
+      RunningTask = true;
     } // unlock Mutex
 
     {
@@ -763,7 +766,7 @@
     bool IsEmpty = false;
     {
       std::lock_guard<std::mutex> Lock(Mutex);
-      Requests.pop_front();
+      RunningTask = false;
       IsEmpty = Requests.empty();
     }
     if (IsEmpty)
@@ -843,7 +846,8 @@
 
 bool ASTWorker::blockUntilIdle(Deadline Timeout) const {
   std::unique_lock<std::mutex> Lock(Mutex);
-  return wait(Lock, RequestsCV, Timeout, [&] { return Requests.empty(); });
+  return wait(Lock, RequestsCV, Timeout,
+              [&] { return Requests.empty() && !RunningTask; });
 }
 
 // Render a TUAction to a user-facing string representation.
diff --git a/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp b/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
--- a/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
+++ b/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
@@ -373,10 +373,17 @@
   std::atomic<int> Builds(0), Actions(0);
 
   Notification Start;
-  updateWithDiags(S, Path, "a", WantDiagnostics::Yes, [&](std::vector<Diag>) {
-    ++Builds;
-    Start.wait();
-  });
+  updateWithDiags(S, Path, "b", WantDiagnostics::Auto,
+                  [&](std::vector<Diag>) { ++Builds; });
+  ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(10)));
+  S.runWithAST(
+      "invalidatable-but-running", Path,
+      [&](llvm::Expected<InputsAndAST> AST) {
+        Start.wait();
+        ++Actions;
+        EXPECT_TRUE(bool(AST)) << "Shouldn't be invalidated, because running.";
+      },
+      TUScheduler::InvalidateOnUpdate);
   S.runWithAST(
       "invalidatable", Path,
       [&](llvm::Expected<InputsAndAST> AST) {
@@ -421,7 +428,7 @@
   ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(10)));
 
   EXPECT_EQ(2, Builds.load()) << "Middle build should be skipped";
-  EXPECT_EQ(4, Actions.load()) << "All actions should run (some with error)";
+  EXPECT_EQ(5, Actions.load()) << "All actions should run (some with error)";
 }
 
 TEST_F(TUSchedulerTests, ManyUpdates) {