diff --git a/src/vs/editor/common/diff/diffComputer.ts b/src/vs/editor/common/diff/diffComputer.ts
index 111e90f84784..9a8274f23263 100644
--- a/src/vs/editor/common/diff/diffComputer.ts
+++ b/src/vs/editor/common/diff/diffComputer.ts
@@ -74,16 +74,17 @@ class LineMarkerSequence implements ISequence {
 		return r + 2;
 	}
 
-	public getCharSequence(startIndex: number, endIndex: number): CharSequence {
+	public getCharSequence(shouldIgnoreTrimWhitespace: boolean, startIndex: number, endIndex: number): CharSequence {
 		let charCodes: number[] = [];
 		let lineNumbers: number[] = [];
 		let columns: number[] = [];
 		let len = 0;
 		for (let index = startIndex; index <= endIndex; index++) {
-			const startColumn = this._startColumns[index];
-			const endColumn = this._endColumns[index];
+			const lineContent = this._lines[index];
+			const startColumn = (shouldIgnoreTrimWhitespace ? this._startColumns[index] : 1);
+			const endColumn = (shouldIgnoreTrimWhitespace ? this._endColumns[index] : lineContent.length + 1);
 			for (let col = startColumn; col < endColumn; col++) {
-				charCodes[len] = this._lines[index].charCodeAt(col - 1);
+				charCodes[len] = lineContent.charCodeAt(col - 1);
 				lineNumbers[len] = index + 1;
 				columns[len] = col;
 				len++;
@@ -254,7 +255,7 @@ class LineChange implements ILineChange {
 		this.charChanges = charChanges;
 	}
 
-	public static createFromDiffResult(diffChange: IDiffChange, originalLineSequence: LineMarkerSequence, modifiedLineSequence: LineMarkerSequence, continueProcessingPredicate: () => boolean, shouldPostProcessCharChanges: boolean): LineChange {
+	public static createFromDiffResult(shouldIgnoreTrimWhitespace: boolean, diffChange: IDiffChange, originalLineSequence: LineMarkerSequence, modifiedLineSequence: LineMarkerSequence, continueProcessingPredicate: () => boolean, shouldPostProcessCharChanges: boolean): LineChange {
 		let originalStartLineNumber: number;
 		let originalEndLineNumber: number;
 		let modifiedStartLineNumber: number;
@@ -278,8 +279,8 @@ class LineChange implements ILineChange {
 		}
 
 		if (diffChange.originalLength !== 0 && diffChange.modifiedLength !== 0 && continueProcessingPredicate()) {
-			const originalCharSequence = originalLineSequence.getCharSequence(diffChange.originalStart, diffChange.originalStart + diffChange.originalLength - 1);
-			const modifiedCharSequence = modifiedLineSequence.getCharSequence(diffChange.modifiedStart, diffChange.modifiedStart + diffChange.modifiedLength - 1);
+			const originalCharSequence = originalLineSequence.getCharSequence(shouldIgnoreTrimWhitespace, diffChange.originalStart, diffChange.originalStart + diffChange.originalLength - 1);
+			const modifiedCharSequence = modifiedLineSequence.getCharSequence(shouldIgnoreTrimWhitespace, diffChange.modifiedStart, diffChange.modifiedStart + diffChange.modifiedLength - 1);
 
 			let rawChanges = computeDiff(originalCharSequence, modifiedCharSequence, continueProcessingPredicate, true);
 
@@ -379,7 +380,7 @@ export class DiffComputer {
 		if (this.shouldIgnoreTrimWhitespace) {
 			let lineChanges: LineChange[] = [];
 			for (let i = 0, length = rawChanges.length; i < length; i++) {
-				lineChanges.push(LineChange.createFromDiffResult(rawChanges[i], this.original, this.modified, this._continueProcessingPredicate.bind(this), this.shouldPostProcessCharChanges));
+				lineChanges.push(LineChange.createFromDiffResult(this.shouldIgnoreTrimWhitespace, rawChanges[i], this.original, this.modified, this._continueProcessingPredicate.bind(this), this.shouldPostProcessCharChanges));
 			}
 			return lineChanges;
 		}
@@ -454,7 +455,7 @@ export class DiffComputer {
 
 			if (nextChange) {
 				// Emit the actual change
-				result.push(LineChange.createFromDiffResult(nextChange, this.original, this.modified, this._continueProcessingPredicate.bind(this), this.shouldPostProcessCharChanges));
+				result.push(LineChange.createFromDiffResult(this.shouldIgnoreTrimWhitespace, nextChange, this.original, this.modified, this._continueProcessingPredicate.bind(this), this.shouldPostProcessCharChanges));
 
 				originalLineIndex += nextChange.originalLength;
 				modifiedLineIndex += nextChange.modifiedLength;
diff --git a/src/vs/editor/test/common/diff/diffComputer.test.ts b/src/vs/editor/test/common/diff/diffComputer.test.ts
index db7febae627c..2a7cc783ee3a 100644
--- a/src/vs/editor/test/common/diff/diffComputer.test.ts
+++ b/src/vs/editor/test/common/diff/diffComputer.test.ts
@@ -679,7 +679,7 @@ suite('Editor Diff - DiffComputer', () => {
 			' * `yarn [install]` -- Install project NPM dependencies. This is automatically done when you first create the project. You should only need to run this if you add dependencies in `package.json`.',
 		];
 		let modified = [
-			'  * `yarn` -- Install project NPM dependencies. You should only need to run this if you add dependencies in `package.json`.',
+			' * `yarn` -- Install project NPM dependencies. You should only need to run this if you add dependencies in `package.json`.',
 		];
 		let expected = [
 			createLineChange(
@@ -692,4 +692,24 @@ suite('Editor Diff - DiffComputer', () => {
 		];
 		assertDiff(original, modified, expected, true, false);
 	});
+
+	test('issue #42751', () => {
+		let original = [
+			'    1',
+			'  2',
+		];
+		let modified = [
+			'    1',
+			'   3',
+		];
+		let expected = [
+			createLineChange(
+				2, 2, 2, 2,
+				[
+					createCharChange(2, 3, 2, 4, 2, 3, 2, 5)
+				]
+			)
+		];
+		assertDiff(original, modified, expected, true, false);
+	});
 });