fix: checkpoint restore button showing empty floating window

- Fix issue where checkpoint restore button shows empty popover content
- When portalContainer is undefined, PopoverContent now renders normally without portal
- Only pass container prop when portalContainer is available
- Resolves issue #8646

Author: roomote

webview-ui/src/components/chat/checkpoints/CheckpointMenu.tsx

@@ -86,7 +86,7 @@ export const CheckpointMenu = ({
 						</Button>
 					</PopoverTrigger>
 				</StandardTooltip>
-				<PopoverContent align="end" container={portalContainer}>
+				<PopoverContent align="end" {...(portalContainer && { container: portalContainer })}>
 					<div className="flex flex-col gap-2">
 						{!isCurrent && (
 							<div className="flex flex-col gap-1 group hover:text-foreground">