chore: minor cleanup

frontend/src/lib/uPlotV2/components/Tooltip/Tooltip.tsx

@@ -24,10 +24,15 @@ export default function Tooltip({
 	);
 
 	const showHeader = showTooltipHeader || activeItem != null;
-	// With a single series the active item is fully represented in the header —
-	// hide the divider and list to avoid showing a duplicate row.
-	const showList = tooltipContent.length > 1;
-	const showDivider = showList && showHeader;
+	// A single row collapses into the header when it's the active item, but
+	// must stay in the list when there's no active item (e.g. sync-driven
+	// tooltips with no focused series) — otherwise the row would vanish.
+	const showList =
+		tooltipContent.length > 1 ||
+		(tooltipContent.length === 1 && activeItem == null);
+	// The divider separates the active row in the header from the list; with
+	// no active item it has nothing to separate.
+	const showDivider = showList && showHeader && activeItem != null;
 
 	return (
 		<div

frontend/src/lib/uPlotV2/plugins/TooltipPlugin/syncDisplayHook.ts

@@ -137,7 +137,7 @@ function applyReceiverSync({
 
 	if (commonKeys.length === 0) {
 		uPlotInstance.setSeries(null, { focus: false });
-		return [];
+		return noMatchResult;
 	}
 
 	if ((uPlotInstance.cursor.left ?? -1) < 0) {