Don't open two browser tabs when clicking the pull request number

webviews/components/header.tsx

@@ -136,7 +136,10 @@ function Title({ title, titleHTML, number, url, inEditMode, setEditMode, setCurr
 					title={url}
 					data-vscode-context={JSON.stringify(context)}
 					onClick={event => {
+						// The webview host opens any anchor with an href and ignores defaultPrevented,
+						// so the click must not reach it or a second browser tab is opened.
 						event.preventDefault();
+						event.stopPropagation();
 						void openOnGitHub();
 					}}
 				>

webviews/components/stickyHeader.tsx

@@ -65,7 +65,10 @@ export function StickyHeader({ pr, visible }: { pr: PullRequest; visible: boolea
 						'github:copyMenu': true,
 					})}
 					onClick={event => {
+						// The webview host opens any anchor with an href and ignores defaultPrevented,
+						// so the click must not reach it or a second browser tab is opened.
 						event.preventDefault();
+						event.stopPropagation();
 						void openOnGitHub();
 					}}
 				>

webviews/editorWebview/test/overview.test.tsx

@@ -59,6 +59,45 @@ describe('Overview', function () {
 		assert.strictEqual(openOnGitHub.callCount, 2);
 	});
 
+	it('opens a PR number link exactly once', function () {
+		const pr = new PullRequestBuilder().build();
+		const context = new PRContext(pr);
+		const openOnGitHub = sinon.stub(context, 'openOnGitHub');
+
+		// Stands in for the webview host, which opens any anchor with an href that a click
+		// reaches, and does not check defaultPrevented.
+		const hostOpenedLinks: string[] = [];
+		const hostLinkHandler = (event: Event) => {
+			const anchor = (event.target as HTMLElement).closest('a[href]');
+			if (anchor) {
+				hostOpenedLinks.push(anchor.getAttribute('href')!);
+			}
+		};
+		window.addEventListener('click', hostLinkHandler);
+
+		try {
+			const out = render(
+				<PullRequestContext.Provider value={context}>
+					<Overview {...pr} />
+				</PullRequestContext.Provider>,
+			);
+
+			const numberLinks = out.container.querySelectorAll('.overview-title a, .sticky-header-number');
+			assert.strictEqual(numberLinks.length, 2);
+			numberLinks.forEach(link => {
+				openOnGitHub.resetHistory();
+				hostOpenedLinks.length = 0;
+
+				fireEvent.click(link);
+
+				assert.strictEqual(openOnGitHub.callCount, 1);
+				assert.deepStrictEqual(hostOpenedLinks, []);
+			});
+		} finally {
+			window.removeEventListener('click', hostLinkHandler);
+		}
+	});
+
 	it('shows view changes in both headers', function () {
 		const pr = new PullRequestBuilder().isAgentSessionsWorkspace(true).build();
 		const context = new PRContext(pr);