Use zustand for state management

frontend/package-lock.json

@@ -12,7 +12,8 @@
         "@tanstack/react-query-devtools": "^5.74.6",
         "react": "^19.0.0",
         "react-dom": "^19.0.0",
-        "react-router-dom": "^7.5.1"
+        "react-router-dom": "^7.5.1",
+        "zustand": "^5.0.3"
       },
       "devDependencies": {
         "@eslint/js": "^9.22.0",
@@ -1471,7 +1472,7 @@
       "version": "19.1.2",
       "resolved": "https://registry.npmjs.org/@types/react/-/react-19.1.2.tgz",
       "integrity": "sha512-oxLPMytKchWGbnQM9O7D67uPa9paTNxO7jVoNMXgkkErULBPhPARCfkKL9ytcIJJRGjbsVwW4ugJzyFFvm/Tiw==",
-      "dev": true,
+      "devOptional": true,
       "license": "MIT",
       "dependencies": {
         "csstype": "^3.0.2"
@@ -1963,7 +1964,7 @@
       "version": "3.1.3",
       "resolved": "https://registry.npmjs.org/csstype/-/csstype-3.1.3.tgz",
       "integrity": "sha512-M1uQkMl8rQK/szD0LNhtqxIPLpimGm8sOBwU7lLnCpSbTyY3yeU1Vc7l4KT5zT4s/yOxHH5O7tIuuLOCnLADRw==",
-      "dev": true,
+      "devOptional": true,
       "license": "MIT"
     },
     "node_modules/debug": {
@@ -3436,6 +3437,35 @@
       "funding": {
         "url": "https://github.com/sponsors/sindresorhus"
       }
+    },
+    "node_modules/zustand": {
+      "version": "5.0.3",
+      "resolved": "https://registry.npmjs.org/zustand/-/zustand-5.0.3.tgz",
+      "integrity": "sha512-14fwWQtU3pH4dE0dOpdMiWjddcH+QzKIgk1cl8epwSE7yag43k/AD/m4L6+K7DytAOr9gGBe3/EXj9g7cdostg==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=12.20.0"
+      },
+      "peerDependencies": {
+        "@types/react": ">=18.0.0",
+        "immer": ">=9.0.6",
+        "react": ">=18.0.0",
+        "use-sync-external-store": ">=1.2.0"
+      },
+      "peerDependenciesMeta": {
+        "@types/react": {
+          "optional": true
+        },
+        "immer": {
+          "optional": true
+        },
+        "react": {
+          "optional": true
+        },
+        "use-sync-external-store": {
+          "optional": true
+        }
+      }
     }
   }
 }

frontend/package.json

@@ -14,7 +14,8 @@
     "@tanstack/react-query-devtools": "^5.74.6",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
-    "react-router-dom": "^7.5.1"
+    "react-router-dom": "^7.5.1",
+    "zustand": "^5.0.3"
   },
   "devDependencies": {
     "@eslint/js": "^9.22.0",

frontend/src/App.tsx

@@ -1,14 +1,17 @@
-import { useState } from "react";
-import Panel from "./components/Panel";
-
 import { useQueries } from "@tanstack/react-query";
 import { useNavigate } from "react-router-dom";
-import "./App.css";
-import ArticleSelector from "./components/ArticleSelector";
-import TOC from "./components/TOC";
+
 import { getArticleIds, getToc } from "./lib/api";
 import { Language } from "./lib/types";
 
+import ArticleSelector from "./components/ArticleSelector";
+import Panel from "./components/Panel";
+import TOC from "./components/TOC";
+
+import useUIStore from "./store/uiStore";
+
+import "./App.css";
+
 type Props = {
   celexId: string;
   articleId: number;
@@ -16,10 +19,7 @@ type Props = {
 
 function App({ celexId, articleId }: Props) {
   const navigate = useNavigate();
-  const [numPanels, setNumPanels] = useState(1);
-  const [selectedParagraphId, setSelectedParagraphId] = useState<string | null>(
-    null
-  );
+  const { numPanels, addPanel } = useUIStore();
 
   const results = useQueries({
     queries: [
@@ -76,9 +76,7 @@ function App({ celexId, articleId }: Props) {
           selectedId={articleId}
           onSelected={(id) => navigate(`/${celexId}/articles/${id}`)}
         />
-        <button onClick={() => setNumPanels((prev) => prev + 1)}>
-          Add Panel
-        </button>
+        <button onClick={addPanel}>Add Panel</button>
       </div>
       <div className="panel-container">
         <TOC
@@ -94,8 +92,6 @@ function App({ celexId, articleId }: Props) {
               Object.values(Language)[index % Object.values(Language).length]
             }
             articleId={articleId}
-            selectedParagraphId={selectedParagraphId || undefined}
-            onParagraphSelected={setSelectedParagraphId}
           />
         ))}
       </div>

frontend/src/components/Panel.tsx

@@ -2,6 +2,7 @@ import { useQuery } from "@tanstack/react-query";
 import { useEffect, useRef, useState } from "react";
 import { getArticle } from "../lib/api";
 import { Language } from "../lib/types";
+import useUIStore from "../store/uiStore";
 import LanguageSwitcher from "./LanguageSwitcher";
 import "./Panel.css";
 
@@ -9,17 +10,11 @@ type PanelProps = {
   celexId: string;
   language?: Language;
   articleId: number;
-  selectedParagraphId?: string;
-  onParagraphSelected(paragraphId: string): void;
 };
 
-function Panel({
-  celexId,
-  language,
-  articleId,
-  onParagraphSelected,
-  selectedParagraphId,
-}: PanelProps) {
+function Panel({ celexId, language, articleId }: PanelProps) {
+  const { selectedParagraphId, setSelectedParagraphId } = useUIStore();
+
   const [lang, setLang] = useState(language || Language.ENG);
   const articleRef = useRef<HTMLDivElement>(null);
   const { data, isPending, error } = useQuery({
@@ -39,11 +34,11 @@ function Panel({
       p.classList.remove("highlight");
     });
     if (selectedParagraphId) {
-      const selectedParagraph = Array.from(paragraphs).find(
+      const el = Array.from(paragraphs).find(
         (p) => p.getAttribute("data-paragraph-id") === selectedParagraphId
       );
-      if (selectedParagraph) {
-        selectedParagraph.classList.add("highlight");
+      if (el) {
+        el.classList.add("highlight");
       }
     }
 
@@ -58,7 +53,7 @@ function Panel({
         return;
       }
       target.classList.add("highlight");
-      onParagraphSelected(paragraphId);
+      setSelectedParagraphId(paragraphId);
     };
 
     paragraphs.forEach((element) => {
@@ -72,7 +67,7 @@ function Panel({
         element.removeEventListener("click", handleClick(element));
       });
     };
-  }, [articleRef, data, selectedParagraphId, onParagraphSelected]);
+  }, [articleRef, data, selectedParagraphId, setSelectedParagraphId]);
 
   if (isPending) return "Loading...";
   if (error) return "An error has occurred: " + error.message;

frontend/src/store/uiStore.ts

@@ -0,0 +1,21 @@
+import { create } from "zustand";
+
+interface UIState {
+  numPanels: number;
+  addPanel: () => void;
+  removePanel: () => void;
+
+  selectedParagraphId: string | null;
+  setSelectedParagraphId: (selectedParagraphId: string | null) => void;
+}
+const useUIStore = create<UIState>((set) => ({
+  numPanels: 1,
+  selectedParagraphId: null,
+  addPanel: () => set((state) => ({ numPanels: state.numPanels + 1 })),
+  removePanel: () =>
+    set((state) => ({ numPanels: Math.max(state.numPanels - 1, 1) })),
+  setSelectedParagraphId: (selectedParagraphId: string | null) =>
+    set({ selectedParagraphId }),
+}));
+
+export default useUIStore;