Fix issue when areas with 0 tokens would be used for delta encoding

src/vs/workbench/api/common/extHostLanguageFeatures.ts

@@ -746,6 +746,10 @@ export class SemanticColoringAdapter {
 		for (let i = 0, len = oldAreas.length; i < len; i++) {
 			const oldAreaData = oldAreas[i];
 			const oldAreaTokenCount = (oldAreaData.length / 5) | 0;
+			if (oldAreaTokenCount === 0) {
+				// skip old empty areas
+				continue;
+			}
 			if (newTokenEndIndex - newTokenStartIndex < oldAreaTokenCount) {
 				// there are too many old tokens, this cannot work
 				break;
@@ -772,6 +776,10 @@ export class SemanticColoringAdapter {
 		for (let i = oldAreas.length - 1; i >= 0; i--) {
 			const oldAreaData = oldAreas[i];
 			const oldAreaTokenCount = (oldAreaData.length / 5) | 0;
+			if (oldAreaTokenCount === 0) {
+				// skip old empty areas
+				continue;
+			}
 			if (newTokenEndIndex - newTokenStartIndex < oldAreaTokenCount) {
 				// there are too many old tokens, this cannot work
 				break;

src/vs/workbench/test/electron-browser/api/semanticTokens.test.ts

@@ -6,7 +6,7 @@
 import * as assert from 'assert';
 import { URI } from 'vs/base/common/uri';
 import * as types from 'vs/workbench/api/common/extHostTypes';
-import { TestRPCProtocol } from './testRPCProtocol';
+import { TestRPCProtocol } from 'vs/workbench/test/electron-browser/api/testRPCProtocol';
 import { SemanticColoringAdapter } from 'vs/workbench/api/common/extHostLanguageFeatures';
 import { ExtHostDocuments } from 'vs/workbench/api/common/extHostDocuments';
 import { ExtHostDocumentsAndEditors } from 'vs/workbench/api/common/extHostDocumentsAndEditors';
@@ -305,4 +305,39 @@ suite('SemanticColoringAdapter', () => {
 		]);
 		adapter.releaseSemanticColoring(result1.id);
 	});
+
+	test('going from empty to 1 semantic token', async () => {
+		doc.onEvents({
+			versionId: 2,
+			eol: '\n',
+			changes: [{
+				range: { startLineNumber: 1, startColumn: 1, endLineNumber: 23, endColumn: 18 },
+				rangeOffset: 0,
+				rangeLength: 0,
+				text: ''
+			}]
+		});
+
+		const result1 = decodeSemanticTokensDto((await adapter.provideSemanticColoring(resource, 0, CancellationToken.None))!);
+		assertDTO(result1, [
+			{ type: 'full', line: 1, tokens: [] },
+		]);
+
+		doc.onEvents({
+			versionId: 3,
+			eol: '\n',
+			changes: [{
+				range: { startLineNumber: 1, startColumn: 1, endLineNumber: 1, endColumn: 1 },
+				rangeOffset: 0,
+				rangeLength: 0,
+				text: 'const enum E01 {}\n'
+			}]
+		});
+
+		const result2 = decodeSemanticTokensDto((await adapter.provideSemanticColoring(resource, result1.id, CancellationToken.None))!);
+		assertDTO(result2, [
+			{ type: 'full', line: 1, tokens: [0] }
+		]);
+		adapter.releaseSemanticColoring(result1.id);
+	});
 });