Allow checked out PRs to show diffs only against PR head

Fixes #4720

package.json

@@ -947,6 +947,12 @@
 				"category": "%command.pull.request.category%",
 				"icon": "$(comment)"
 			},
+			{
+				"command": "review.diffWithPrHead",
+				"title": "%command.review.diffWithPrHead.title%",
+				"category": "%command.pull.request.category%",
+				"icon": "$(cloud)"
+			},
 			{
 				"command": "issue.createIssueFromSelection",
 				"title": "%command.issue.createIssueFromSelection.title%",
@@ -1469,6 +1475,10 @@
 					"command": "pr.addFileComment",
 					"when": "false"
 				},
+				{
+					"command": "review.diffWithPrHead",
+					"when": "false"
+				},
 				{
 					"command": "issue.copyGithubPermalink",
 					"when": "github:hasGitHubRemotes"
@@ -1758,6 +1768,11 @@
 					"group": "inline@0",
 					"when": "!openDiffOnClick && view == prStatus:github && viewItem =~ /filechange(?!:DELETE)/"
 				},
+				{
+					"command": "review.diffWithPrHead",
+					"group": "inline@1",
+					"when": "openDiffOnClick && view == prStatus:github && viewItem =~ /filechange(?!:DELETE)/"
+				},
 				{
 					"command": "pr.openOriginalFile",
 					"when": "view =~ /(pr|prStatus):github/ && viewItem =~ /filechange:MODIFY/"

package.nls.json

@@ -184,6 +184,7 @@
 	"command.pr.applySuggestion.title": "Apply Suggestion",
 	"command.pr.addLabelsToNewPr.title": "Add Labels",
 	"command.pr.addFileComment.title": "Add File Comment",
+	"command.review.diffWithPrHead.title": "Open Diff With Pull Request Head (readonly)",
 	"command.issues.category": "GitHub Issues",
 	"command.issue.createIssueFromSelection.title": "Create Issue From Selection",
 	"command.issue.createIssueFromClipboard.title": "Create Issue From Clipboard",

src/commands.ts

@@ -13,7 +13,7 @@ import { IComment } from './common/comment';
 import Logger from './common/logger';
 import { FILE_LIST_LAYOUT, PR_SETTINGS_NAMESPACE } from './common/settingKeys';
 import { ITelemetry } from './common/telemetry';
-import { asImageDataURI, fromReviewUri, Schemes } from './common/uri';
+import { asImageDataURI, fromReviewUri, Schemes, toPRUri } from './common/uri';
 import { formatError } from './common/utils';
 import { EXTENSION_ID } from './constants';
 import { FolderRepositoryManager } from './github/folderRepositoryManager';
@@ -1242,6 +1242,28 @@ ${contents}
 			return vscode.commands.executeCommand('workbench.action.addComment', { fileComment: true });
 		}));
 
+	context.subscriptions.push(
+		vscode.commands.registerCommand('review.diffWithPrHead', async (fileChangeNode: GitFileChangeNode) => {
+			const fileName = fileChangeNode.fileName;
+			let parentURI = toPRUri(
+				fileChangeNode.resourceUri,
+				fileChangeNode.pullRequest,
+				fileChangeNode.pullRequest.base.sha,
+				fileChangeNode.pullRequest.head.sha,
+				fileChangeNode.fileName,
+				true,
+				fileChangeNode.status);
+			let headURI = toPRUri(
+				fileChangeNode.resourceUri,
+				fileChangeNode.pullRequest,
+				fileChangeNode.pullRequest.base.sha,
+				fileChangeNode.pullRequest.head.sha,
+				fileChangeNode.fileName,
+				false,
+				fileChangeNode.status);
+			return vscode.commands.executeCommand('vscode.diff', parentURI, headURI, `${fileName} (Pull Request Diff with Head)`);
+		}));
+
 	function goToNextPrevDiff(diffs: vscode.LineChange[], next: boolean) {
 		const tab = vscode.window.tabGroups.activeTabGroup.activeTab;
 		const input = tab?.input;

src/view/reviewManager.ts

@@ -34,8 +34,9 @@ import { GithubItemStateEnum } from '../github/interface';
 import { PullRequestGitHelper, PullRequestMetadata } from '../github/pullRequestGitHelper';
 import { IResolvedPullRequestModel, PullRequestModel } from '../github/pullRequestModel';
 import { CreatePullRequestHelper } from './createPullRequestHelper';
-import { GitFileChangeModel } from './fileChangeModel';
+import { GitFileChangeModel, InMemFileChangeModel, RemoteFileChangeModel } from './fileChangeModel';
 import { getGitHubFileContent } from './gitHubContentProvider';
+import { getInMemPRFileSystemProvider, provideDocumentContentForChangeModel } from './inMemPRContentProvider';
 import { PullRequestChangesTreeDataProvider } from './prChangesTreeDataProvider';
 import { ProgressHelper } from './progress';
 import { RemoteQuickPickItem } from './quickpick';
@@ -55,6 +56,7 @@ export class ReviewManager {
 	private _validateStatusInProgress?: Promise<void>;
 	private _reviewCommentController: ReviewCommentController | undefined;
 	private _quickDiffProvider: vscode.Disposable | undefined;
+	private _inMemGitHubContentProvider: vscode.Disposable | undefined;
 
 	private _statusBarItem: vscode.StatusBarItem;
 	private _prNumber?: number;
@@ -445,6 +447,7 @@ export class ReviewManager {
 				this._changesSinceLastReviewProgress.endProgress();
 			})
 		);
+		await this.registerGitHubInMemContentProvider();
 
 		this.statusBarItem.text = '$(git-pull-request) ' + vscode.l10n.t('Pull Request #{0}', pr.number);
 		this.statusBarItem.command = {
@@ -733,6 +736,50 @@ export class ReviewManager {
 		}
 	}
 
+	private async registerGitHubInMemContentProvider() {
+		this._inMemGitHubContentProvider?.dispose();
+		this._inMemGitHubContentProvider = undefined;
+
+		const pr = this._folderRepoManager.activePullRequest;
+		if (!pr) {
+			return;
+		}
+		const rawChanges = await pr.getFileChangesInfo();
+		const mergeBase = pr.mergeBase;
+		if (!mergeBase) {
+			return;
+		}
+		const changes = rawChanges.map(change => {
+			if (change instanceof SlimFileChange) {
+				return new RemoteFileChangeModel(this._folderRepoManager, change, pr);
+			}
+			return new InMemFileChangeModel(this._folderRepoManager,
+				pr as (PullRequestModel & IResolvedPullRequestModel),
+				change, true, mergeBase);
+		});
+
+		this._inMemGitHubContentProvider = getInMemPRFileSystemProvider()?.registerTextDocumentContentProvider(
+			pr.number,
+			async (uri: vscode.Uri): Promise<string> => {
+				const params = fromPRUri(uri);
+				if (!params) {
+					return '';
+				}
+				const fileChange = changes.find(
+					contentChange => contentChange.fileName === params.fileName,
+				);
+
+				if (!fileChange) {
+					Logger.error(`Cannot find content for document ${uri.toString()}`, 'PR');
+					return '';
+				}
+
+				return provideDocumentContentForChangeModel(this._folderRepoManager, pr, params, fileChange);
+
+			},
+		);
+	}
+
 	private async registerCommentController() {
 		if (this._folderRepoManager.activePullRequest?.reviewThreadsCacheReady && this._reviewModel.hasLocalFileChanges) {
 			await this.doRegisterCommentController();
@@ -1082,6 +1129,8 @@ export class ReviewManager {
 
 		this._reviewCommentController?.dispose();
 		this._reviewCommentController = undefined;
+		this._inMemGitHubContentProvider?.dispose();
+		this._inMemGitHubContentProvider = undefined;
 	}
 
 	async provideTextDocumentContent(uri: vscode.Uri): Promise<string | undefined> {