Make sure we render border in split webviews

src/vs/workbench/contrib/customEditor/browser/customEditors.ts

@@ -180,7 +180,7 @@ export class CustomEditorService extends Disposable implements ICustomEditorServ
 	): CustomFileEditorInput {
 		const id = generateUuid();
 		const webview = new Lazy(() => {
-			return new UnownedDisposable(this.webviewService.createWebviewEditorOverlay(id, { customClasses: options ? options.customClasses : undefined }, {}));
+			return new UnownedDisposable(this.webviewService.createWebviewEditorOverlay(id, { customClasses: options?.customClasses }, {}));
 		});
 		const input = this.instantiationService.createInstance(CustomFileEditorInput, resource, viewType, id, webview);
 		if (group) {

src/vs/workbench/contrib/webview/browser/dynamicWebviewEditorOverlay.ts

@@ -143,7 +143,7 @@ export class DynamicWebviewEditorOverlay extends Disposable implements WebviewEd
 	}
 
 	public get options(): WebviewOptions { return this._options; }
-	public set options(value: WebviewOptions) { this._options = value; }
+	public set options(value: WebviewOptions) { this._options = { customClasses: this._options.customClasses, ...value }; }
 
 	public get contentOptions(): WebviewContentOptions { return this._contentOptions; }
 	public set contentOptions(value: WebviewContentOptions) {

src/vs/workbench/contrib/webview/browser/webviewElement.ts

@@ -60,7 +60,7 @@ export class IFrameWebview extends BaseWebview<HTMLIFrameElement> implements Web
 
 	protected createElement(options: WebviewOptions) {
 		const element = document.createElement('iframe');
-		element.className = `webview ${options.customClasses}`;
+		element.className = `webview ${options.customClasses || ''}`;
 		element.sandbox.add('allow-scripts', 'allow-same-origin');
 		element.setAttribute('src', `${this.externalEndpoint}/index.html?id=${this.id}`);
 		element.style.border = 'none';

src/vs/workbench/contrib/webview/electron-browser/webviewElement.ts

@@ -296,7 +296,7 @@ export class ElectronWebviewBasedWebview extends BaseWebview<WebviewTag> impleme
 		const element = document.createElement('webview');
 		element.setAttribute('partition', `webview${Date.now()}`);
 		element.setAttribute('webpreferences', 'contextIsolation=yes');
-		element.className = `webview ${options.customClasses}`;
+		element.className = `webview ${options.customClasses || ''}`;
 
 		element.style.flex = '0 1';
 		element.style.width = '0';