only clear combiner if 1 item

client/src/components/vbox.component.jsx

@@ -333,7 +333,9 @@ class Vbox extends preact.Component {
                 <label
                     draggable='true'
                     onDragStart={ev => ev.dataTransfer.setData('text', '')}
-                    onDragEnd={() => combinerChange([])}>
+                    onDragEnd={() => {
+                        if (combiner.length === 1) combinerChange([]);
+                    }}>
                     <button
                         class={classes}
                         onMouseOver={e => vboxHover(e, v)}