Shift showSidePanel to URL as source of truth

docs/APP_REFACTORING_DESIGN.md

@@ -96,7 +96,7 @@ We will gradually eliminate React state variables in `src/app.tsx` and derive th
 *   [x] **Step 3.1c:** Shift `showWikiTreeMenus` to URL as SSOT. Derive it directly from search parameters on render and remove its React state variable.
 *   [x] **Step 3.1d:** Shift `freezeAnimation` to URL as SSOT. Derive it directly from search parameters on render and remove its React state variable.
 *   [x] **Step 3.2:** Extract `selection` and `detailIndi` state. Parse them directly from URL params (`indi`, `gen`, and `detail`); update display selectors. Remove React states. Ensure that chart selection changes (`onSelection` callback) explicitly clear or update the `detail` query parameter to match the new selection to avoid getting stuck on the old details viewport. Also update the detail-only selection handler (`onDetailSelection`) to update the `detail` query parameter in the URL.
-*   [ ] **Step 3.3:** Extract `showSidePanel` state. Derive state directly from `?sidePanel=` parameter. Remove React state. Enhance URL args helpers to allow generating path/query target objects with replaced values, and ensure layout settings (`sidePanel`) and configuration changes use `replace: true` to prevent polluting the browser history stack.
+*   [x] **Step 3.3:** Extract `showSidePanel` state. Derive state directly from `?sidePanel=` parameter. Remove React state. Enhance URL args helpers to allow generating path/query target objects with replaced values, and ensure layout settings (`sidePanel`) and configuration changes use `replace: true` to prevent polluting the browser history stack.
 *   [ ] **Step 3.4:** Extract `config` state. Parse display settings on render using `argsToConfig` helper. Remove state. In `ViewPage`, run `updateChartWithConfig(config, data)` synchronously during the render pass (e.g. in the `useMemo` that derives query parameters/config) to update the in-memory chart data before it is rendered by the `<Chart>` child component. Run E2E and visual tests to verify no rendering regressions occurred.
 
 #### Phase 4: Structural Page Nesting

src/app.tsx

@@ -86,8 +86,6 @@ export function App() {
 
   /** Error to display. */
   const [error, setError] = useState<string>();
-  /** Whether the side panel is shown. */
-  const [showSidePanel, setShowSidePanel] = useState(false);
 
   /** Whether to show the error popup. */
   const [showErrorPopup, setShowErrorPopup] = useState(false);
@@ -122,6 +120,8 @@ export function App() {
   const showWikiTreeMenus = args.showWikiTreeMenus;
   /** Freeze animations after initial chart render. */
   const freezeAnimation = args.freezeAnimation;
+  /** Whether the side panel is shown. */
+  const showSidePanel = args.showSidePanel;
   /** The currently selected individual. Fallback to default individual from loaded data if not specified. */
   const updatedSelection = useMemo(() => {
     return data ? getSelection(data.chartData, args.selection) : undefined;
@@ -161,10 +161,12 @@ export function App() {
 
   function onToggleSidePanel() {
     const newShowSidePanel = !showSidePanel;
-    setShowSidePanel(newShowSidePanel);
+    updateUrl(
-    updateUrl({
+      {
-      sidePanel: newShowSidePanel ? 'true' : 'false',
+        sidePanel: newShowSidePanel ? 'true' : 'false',
-    });
+      },
+      {replace: true},
+    );
   }
 
   /** Sets error message after data load failure. */
@@ -368,7 +370,6 @@ export function App() {
             args.selection,
           );
           updateChartWithConfig(args.config, data);
-          setShowSidePanel(args.showSidePanel);
           setState(AppState.SHOWING_CHART);
         } catch (error: unknown) {
           if (!isMountedRef.current || fetchIdRef.current !== currentFetchId) {
@@ -446,8 +447,9 @@ export function App() {
 
   function updateUrl(
     args: Record<string, string | (string | null)[] | null | undefined>,
+    options?: {replace?: boolean},
   ) {
-    navigate(getUrlForArgs(location, args));
+    navigate(getUrlForArgs(location, args), options);
   }
 
   /**
@@ -587,7 +589,7 @@ export function App() {
                 onConfigChange={(config) => {
                   setConfig(config);
                   updateChartWithConfig(config, data);
-                  updateUrl(configToArgs(config));
+                  updateUrl(configToArgs(config), {replace: true});
                 }}
               />
               <SidebarPusher>{renderChart(selection)}</SidebarPusher>