Fixes microsoft/vscode-internalbacklog#4466

Author: hediet

src/vs/editor/common/diff/algorithms/joinSequenceDiffs.ts

@@ -45,14 +45,16 @@ export function smoothenSequenceDiffs(sequence1: ISequence, sequence2: ISequence
  * Improved diff: [{Add ", Foo" after Bar}]
  */
 export function joinSequenceDiffs(sequence1: ISequence, sequence2: ISequence, sequenceDiffs: SequenceDiff[]): SequenceDiff[] {
-	const result: SequenceDiff[] = [];
-	if (sequenceDiffs.length > 0) {
-		result.push(sequenceDiffs[0]);
+	if (sequenceDiffs.length === 0) {
+		return sequenceDiffs;
 	}
 
+	const result: SequenceDiff[] = [];
+	result.push(sequenceDiffs[0]);
+
 	// First move them all to the left as much as possible and join them if possible
 	for (let i = 1; i < sequenceDiffs.length; i++) {
-		const prevResult = sequenceDiffs[i - 1];
+		const prevResult = result[result.length - 1];
 		let cur = sequenceDiffs[i];
 
 		if (cur.seq1Range.isEmpty || cur.seq2Range.isEmpty) {

src/vs/editor/test/node/diffing/diffingFixture.test.ts

@@ -40,7 +40,8 @@ suite('diff fixtures', () => {
 
 		const diffingAlgo = diffingAlgoName === 'legacy' ? new SmartLinesDiffComputer() : new StandardLinesDiffComputer();
 
-		const diff = diffingAlgo.computeDiff(firstContentLines, secondContentLines, { ignoreTrimWhitespace: false, maxComputationTimeMs: Number.MAX_SAFE_INTEGER, computeMoves: false });
+		const ignoreTrimWhitespace = folder.indexOf('trimws') >= 0;
+		const diff = diffingAlgo.computeDiff(firstContentLines, secondContentLines, { ignoreTrimWhitespace, maxComputationTimeMs: Number.MAX_SAFE_INTEGER, computeMoves: false });
 
 		function getDiffs(changes: readonly LineRangeMapping[]): IDetailedDiff[] {
 			return changes.map<IDetailedDiff>(c => ({
@@ -112,7 +113,7 @@ suite('diff fixtures', () => {
 	}
 
 	test(`test`, () => {
-		runTest('issue-185779', 'advanced');
+		runTest('invalid-diff-trimws', 'advanced');
 	});
 
 	for (const folder of folders) {