Index: clangd/CodeComplete.cpp
===================================================================
--- clangd/CodeComplete.cpp
+++ clangd/CodeComplete.cpp
@@ -40,6 +40,7 @@
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Frontend/FrontendActions.h"
 #include "clang/Index/USRGeneration.h"
+#include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Sema/CodeCompleteConsumer.h"
 #include "clang/Sema/Sema.h"
 #include "clang/Tooling/Core/Replacement.h"
@@ -1053,11 +1054,19 @@
   // We reuse the preamble whether it's valid or not. This is a
   // correctness/performance tradeoff: building without a preamble is slow, and
   // completion is latency-sensitive.
+  // However, if we're completing *inside* the preamble section of the draft,
+  // overriding the preamble will break sema completion. Fortunately we can just
+  // skip all includes in this case; these completions are really simple.
+  bool CompletingInPreamble =
+      ComputePreambleBounds(*CI->getLangOpts(), ContentsBuffer.get(), 0).Size >
+      *Offset;
   // NOTE: we must call BeginSourceFile after prepareCompilerInstance. Otherwise
   // the remapped buffers do not get freed.
   auto Clang = prepareCompilerInstance(
-      std::move(CI), Input.Preamble, std::move(ContentsBuffer),
-      std::move(Input.PCHs), std::move(Input.VFS), DummyDiagsConsumer);
+      std::move(CI), CompletingInPreamble ? nullptr : Input.Preamble,
+      std::move(ContentsBuffer), std::move(Input.PCHs), std::move(Input.VFS),
+      DummyDiagsConsumer);
+  Clang->getPreprocessorOpts().SingleFileParseMode = CompletingInPreamble;
   Clang->setCodeCompletionConsumer(Consumer.release());
 
   SyntaxOnlyAction Action;
Index: unittests/clangd/CodeCompleteTests.cpp
===================================================================
--- unittests/clangd/CodeCompleteTests.cpp
+++ unittests/clangd/CodeCompleteTests.cpp
@@ -657,6 +657,22 @@
               UnorderedElementsAre(Named("local"), Named("preamble")));
 }
 
+// This verifies that we get normal preprocessor completions in the preamble.
+// This is a regression test for an old bug: if we override the preamble and
+// try to complete inside it, clang kicks our completion point just outside the
+// preamble, resulting in always getting top-level completions.
+TEST(CompletionTest, CompletionInPreamble) {
+  EXPECT_THAT(completions(R"cpp(
+    #ifnd^ef FOO_H_
+    #define BAR_H_
+    #include <bar.h>
+    int foo() {}
+    #endif
+    )cpp")
+                  .Completions,
+              ElementsAre(Named("ifndef")));
+};
+
 TEST(CompletionTest, DynamicIndexMultiFile) {
   MockFSProvider FS;
   MockCompilationDatabase CDB;