Use a separate ExecutorService in AsyncCompletionProposalPopup

bundles/org.eclipse.jface.text/src/org/eclipse/jface/text/contentassist/AsyncCompletionProposalPopup.java

@@ -25,6 +25,8 @@
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicInteger;
@@ -70,6 +72,8 @@ class AsyncCompletionProposalPopup extends CompletionProposalPopup {
 
 	private PopupVisibleTimer fPopupVisibleTimer= new PopupVisibleTimer();
 
+	private ExecutorService completionFuturesExecutor;
+
 	private static final class ComputingProposal implements ICompletionProposal, ICompletionProposalExtension {
 
 		private final int fOffset;
@@ -341,6 +345,9 @@ void createProposalSelector() {
 	void cancelFutures() {
 		toCancelFutures.forEach(future -> future.cancel(true));
 		toCancelFutures.clear();
+		if (completionFuturesExecutor != null) {
+			completionFuturesExecutor.shutdownNow();
+		}
 	}
 
 	@Override
@@ -371,6 +378,8 @@ protected List<CompletableFuture<List<ICompletionProposal>>> buildCompletionFutu
 			return Collections.emptyList();
 		}
 		List<CompletableFuture<List<ICompletionProposal>>> futures = new ArrayList<>(processors.size());
+		completionFuturesExecutor= Executors.newSingleThreadExecutor();
+
 		for (IContentAssistProcessor processor : processors) {
 			futures.add(CompletableFuture.supplyAsync(() -> {
 				AtomicReference<List<ICompletionProposal>> result= new AtomicReference<>();
@@ -389,7 +398,7 @@ protected List<CompletableFuture<List<ICompletionProposal>>> buildCompletionFutu
 					return Collections.emptyList();
 				}
 				return proposals;
-			}));
+			}, completionFuturesExecutor));
 		}
 		return futures;
 	}