Re-split library and preview when the editor is hidden; remember the editor's width

src/app/components/PaneToggleStrip.tsx

@@ -51,6 +51,16 @@ export function PaneToggleStrip() {
     preview: previewVisible,
   };
 
+  // The strip's own element — its parent is the panes row, so we can measure the
+  // width available to the panes (row minus this strip) when toggling the editor,
+  // which needs it to remember/restore the editor's width (PanesStore.togglePane).
+  const stripRef = useRef<HTMLDivElement>(null);
+  const panesInner = (): number => {
+    const strip = stripRef.current;
+    const row = strip?.parentElement;
+    return row ? row.clientWidth - strip.offsetWidth : 0;
+  };
+
   // Roving tabindex over the toggles + the trailing Datasets action (the divider
   // is not a control). One tab stop; the focused index is the only `tabIndex=0`.
   const count = PANES.length + 1;
@@ -88,6 +98,7 @@ export function PaneToggleStrip() {
 
   return (
     <div
+      ref={stripRef}
       role="toolbar"
       aria-orientation="vertical"
       aria-label="Workspace panes"
@@ -117,7 +128,7 @@ export function PaneToggleStrip() {
             .filter(Boolean)
             .join(' ')}
           onClick={() => {
-            togglePane(item.pane);
+            togglePane(item.pane, panesInner());
             setFocusIndex(i);
           }}
           onKeyDown={(e) => onKeyDown(e, i)}