feat: add helper header above extensions search

Add a short message above the search box on the Extensions panel. This helps explain the extension divergence to the user.

If they click dismiss, it stores an item in localStorage to prevent the message from showing up on subsequent loads.

Author: jsjoeio

lib/vscode/src/vs/workbench/contrib/extensions/browser/extensionsViewlet.ts

@@ -406,10 +406,44 @@ export class ExtensionsViewPaneContainer extends ViewPaneContainer implements IE
 		this.root = parent;
 
 		const overlay = append(this.root, $('.overlay'));
+
 		const overlayBackgroundColor = this.getColor(SIDE_BAR_DRAG_AND_DROP_BACKGROUND) ?? '';
 		overlay.style.backgroundColor = overlayBackgroundColor;
 		hide(overlay);
 
+		// NOTE@coder this UI element helps users understand the extension marketplace divergence
+		const extensionHelperLocalStorageKey = 'coder.extension-help-message';
+
+		if (localStorage.getItem(extensionHelperLocalStorageKey) === null) {
+			const helperHeader = append(this.root, $('.header'));
+			helperHeader.id = 'codeServerMarketplaceHelper';
+			helperHeader.style.height = 'auto';
+			helperHeader.style.fontWeight = '600';
+			helperHeader.style.padding = 'padding: 5px 16px';
+			helperHeader.style.position = 'relative';
+			helperHeader.innerHTML = `
+			<div style="margin-bottom: 8px;">
+			<p style="margin-bottom: 0; display: flex; align-items: center"><span class="codicon codicon-warning" style="margin-right: 2px; color: #C4A103"></span>WARNING</p>
+			<p style="margin-top: 0; margin-bottom: 4px">
+			These extensions are not official. Find open-source extensions
+			<a href="https://open-vsx.org/" target="_blank">here</a>.
+			See <a href="https://github.com/cdr/code-server/blob/master/doc/FAQ.md#differences-compared-to-vs-code" target="_blank">docs</a>.
+			</p>
+			</div>
+					`;
+
+			const dismiss = append(helperHeader, $('span'));
+			dismiss.innerHTML = 'Dismiss';
+			dismiss.style.display = 'block';
+			dismiss.style.textAlign = 'right';
+			dismiss.style.cursor = 'pointer';
+			dismiss.onclick = () => {
+				// Eventually removes helperHeader from DOM
+				setTimeout(() => this.root?.removeChild(helperHeader));
+				localStorage.setItem(extensionHelperLocalStorageKey, 'viewed');
+			};
+		}
+
 		const header = append(this.root, $('.header'));
 		const placeholder = localize('searchExtensions', "Search Extensions in Marketplace");
 		const searchValue = this.searchViewletState['query.value'] ? this.searchViewletState['query.value'] : '';