Fixes #136540: Do not eat up error or fall back to the next provider in case an error is thrown

src/vs/editor/common/services/getSemanticTokens.ts

@@ -27,6 +27,7 @@ export class DocumentSemanticTokensResult {
 	constructor(
 		public readonly provider: DocumentSemanticTokensProvider,
 		public readonly tokens: SemanticTokens | SemanticTokensEdits | null,
+		public readonly error: any
 	) { }
 }
 
@@ -45,10 +46,11 @@ export async function getDocumentSemanticTokens(model: ITextModel, lastProvider:
 	// Get tokens from all providers at the same time.
 	const results = await Promise.all(providers.map(async (provider) => {
 		let result: SemanticTokens | SemanticTokensEdits | null | undefined;
+		let error: any = null;
 		try {
 			result = await provider.provideDocumentSemanticTokens(model, (provider === lastProvider ? lastResultId : null), token);
 		} catch (err) {
-			onUnexpectedExternalError(err);
+			error = err;
 			result = null;
 		}
 
@@ -56,11 +58,15 @@ export async function getDocumentSemanticTokens(model: ITextModel, lastProvider:
 			result = null;
 		}
 
-		return new DocumentSemanticTokensResult(provider, result);
+		return new DocumentSemanticTokensResult(provider, result, error);
 	}));
 
-	// Try to return the first result with actual tokens
+	// Try to return the first result with actual tokens or
+	// the first result which threw an error (!!)
 	for (const result of results) {
+		if (result.error) {
+			throw result.error;
+		}
 		if (result.tokens) {
 			return result;
 		}

src/vs/editor/test/common/services/getSemanticTokens.test.ts

@@ -0,0 +1,44 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as assert from 'assert';
+import { CancellationToken } from 'vs/base/common/cancellation';
+import { canceled } from 'vs/base/common/errors';
+import { DisposableStore } from 'vs/base/common/lifecycle';
+import { ITextModel } from 'vs/editor/common/model';
+import { DocumentSemanticTokensProvider, DocumentSemanticTokensProviderRegistry, ProviderResult, SemanticTokens, SemanticTokensEdits, SemanticTokensLegend } from 'vs/editor/common/modes';
+import { getDocumentSemanticTokens } from 'vs/editor/common/services/getSemanticTokens';
+import { createTextModel } from 'vs/editor/test/common/editorTestUtils';
+
+suite('getSemanticTokens', () => {
+
+	test('issue #136540: semantic highlighting flickers', async () => {
+		const disposables = new DisposableStore();
+
+		const provider = new class implements DocumentSemanticTokensProvider {
+			getLegend(): SemanticTokensLegend {
+				return { tokenTypes: ['test'], tokenModifiers: [] };
+			}
+			provideDocumentSemanticTokens(model: ITextModel, lastResultId: string | null, token: CancellationToken): ProviderResult<SemanticTokens | SemanticTokensEdits> {
+				throw canceled();
+			}
+			releaseDocumentSemanticTokens(resultId: string | undefined): void {
+			}
+		};
+
+		disposables.add(DocumentSemanticTokensProviderRegistry.register('testLang', provider));
+
+		const textModel = disposables.add(createTextModel('example', undefined, 'testLang'));
+
+		await getDocumentSemanticTokens(textModel, null, null, CancellationToken.None).then((res) => {
+			assert.fail();
+		}, (err) => {
+			assert.ok(!!err);
+		});
+
+		disposables.dispose();
+	});
+
+});