Fixes microsoft#186680 (microsoft#186989)

Author: hediet

src/vs/editor/browser/widget/diffEditorWidget2/diffEditorViewModel.ts

@@ -289,15 +289,18 @@ export class UnchangedRegion {
 			let modStart = mapping.modifiedRange.startLineNumber;
 			let length = mapping.originalRange.length;
 
-			if (origStart === 1 && modStart === 1 && length > minContext + minHiddenLineCount) {
-				if (length < originalLineCount) {
+			const atStart = origStart === 1 && modStart === 1;
+			const atEnd = origStart + length === originalLineCount + 1 && modStart + length === modifiedLineCount + 1;
+
+			if ((atStart || atEnd) && length > minContext + minHiddenLineCount) {
+				if (atStart && !atEnd) {
+					length -= minContext;
+				}
+				if (atEnd && !atStart) {
+					origStart += minContext;
+					modStart += minContext;
 					length -= minContext;
 				}
-				result.push(new UnchangedRegion(origStart, modStart, length, 0, 0));
-			} else if (origStart + length === originalLineCount + 1 && modStart + length === modifiedLineCount + 1 && length > minContext + minHiddenLineCount) {
-				origStart += minContext;
-				modStart += minContext;
-				length -= minContext;
 				result.push(new UnchangedRegion(origStart, modStart, length, 0, 0));
 			} else if (length > minContext * 2 + minHiddenLineCount) {
 				origStart += minContext;

src/vs/editor/test/browser/widget/diffEditorWidget2.test.ts

@@ -0,0 +1,54 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import assert = require('assert');
+import { UnchangedRegion } from 'vs/editor/browser/widget/diffEditorWidget2/diffEditorViewModel';
+import { LineRange } from 'vs/editor/common/core/lineRange';
+import { LineRangeMapping } from 'vs/editor/common/diff/linesDiffComputer';
+
+suite('DiffEditorWidget2', () => {
+	suite('UnchangedRegion', () => {
+		function serialize(regions: UnchangedRegion[]): unknown {
+			return regions.map(r => `${r.originalRange} - ${r.modifiedRange}`);
+		}
+
+		test('Everything changed', () => {
+			assert.deepStrictEqual(serialize(UnchangedRegion.fromDiffs(
+				[new LineRangeMapping(new LineRange(1, 10), new LineRange(1, 10), [])],
+				10,
+				10,
+			)), []);
+		});
+
+		test('Nothing changed', () => {
+			assert.deepStrictEqual(serialize(UnchangedRegion.fromDiffs(
+				[],
+				10,
+				10,
+			)), [
+				"[1,11) - [1,11)"
+			]);
+		});
+
+		test('Change in the middle', () => {
+			assert.deepStrictEqual(serialize(UnchangedRegion.fromDiffs(
+				[new LineRangeMapping(new LineRange(50, 60), new LineRange(50, 60), [])],
+				100,
+				100,
+			)), ([
+				'[1,47) - [1,47)',
+				'[63,101) - [63,101)'
+			]));
+		});
+
+		test('Change at the end', () => {
+			assert.deepStrictEqual(serialize(UnchangedRegion.fromDiffs(
+				[new LineRangeMapping(new LineRange(99, 100), new LineRange(100, 100), [])],
+				100,
+				100,
+			)), (["[1,96) - [1,96)"]));
+		});
+	});
+});