frontend/src/admin/Categories.tsx

@@ -78,6 +78,8 @@ export default function Categories() {
       .finally(() => setLoading(false));
   }
 
+  // Load-on-mount. `load` sets a pending flag before fetching.
+  // eslint-disable-next-line react-hooks/set-state-in-effect
   useEffect(() => { void load(); }, []);
 
   function openNew(parent: number | null) {

frontend/src/admin/Tags.tsx

@@ -36,6 +36,8 @@ export default function Tags() {
       .finally(() => setLoading(false));
   }
 
+  // Load-on-mount. `load` sets a pending flag before fetching.
+  // eslint-disable-next-line react-hooks/set-state-in-effect
   useEffect(() => { void load(); }, []);
 
   function openNew() {

frontend/src/cart/Cart.tsx

@@ -67,6 +67,9 @@ export default function Cart() {
       .finally(() => setLoading(false));
   }
 
+  // Load-on-mount once the session resolves. `loadAll` sets a pending flag
+  // first, which is what the rule sees.
+  // eslint-disable-next-line react-hooks/set-state-in-effect
   useEffect(() => { if (customer) loadAll(); }, [customer]);
 
   // Only ticks while there is something to count down, so an empty cart does
@@ -80,6 +83,10 @@ export default function Cart() {
   // the release rather than sitting at "expiring…" until the page is reloaded.
   useEffect(() => {
     if (!lapsed) return;
+    // Refetching from the server on a tick, which is synchronisation rather
+    // than derived state — the release happens server-side and the client
+    // has no way to know when.
+    // eslint-disable-next-line react-hooks/set-state-in-effect
     loadAll();
     // The header badge counts held items too, so it goes stale in exactly the
     // same way. Safe as a dependency: CartContext memoizes it with an empty

frontend/src/cart/CartContext.tsx

@@ -24,10 +24,16 @@ export function CartProvider({ children }: Readonly<{ children: React.ReactNode
       .catch(() => setItems([]));
   }, []);
 
+  // Synchronising with the session, which is an external system, so an effect
+  // is the right tool. Deriving it instead would push "signed out" onto every
+  // consumer of this context, and remounting on a `key` is more indirection
+  // than the problem deserves. Decided in #99 rather than left to be
+  // re-investigated.
   useEffect(() => {
     if (customer) {
       refresh();
     } else {
+    // eslint-disable-next-line react-hooks/set-state-in-effect
       setItems([]);
     }
   }, [customer, refresh]);

frontend/src/cart/useNow.ts

@@ -25,6 +25,11 @@ export function useNow(intervalMs: number, active: boolean): number {
     if (!active) return;
     // Set immediately as well as on the interval: mounting with `active` already
     // true would otherwise show a value up to intervalMs stale.
+    // Subscribing to the clock, which is an external system — the case the
+    // rule's own documentation names as correct. Set once here as well as on
+    // the interval so a mount with `active` already true is not up to
+    // intervalMs stale.
+    // eslint-disable-next-line react-hooks/set-state-in-effect
     setNow(Date.now());
     const timer = setInterval(() => setNow(Date.now()), intervalMs);
     return () => clearInterval(timer);

frontend/src/customer/CustomerAuthContext.tsx

@@ -33,6 +33,10 @@ export function CustomerAuthProvider({ children }: Readonly<{ children: React.Re
       .finally(() => setLoading(false));
   }, []);
 
+  // `refresh` sets a pending flag before fetching. The rule cannot tell a
+  // fetch with a spinner from a value that was already knowable, and this is
+  // the former.
+  // eslint-disable-next-line react-hooks/set-state-in-effect
   useEffect(() => { refresh(); }, [refresh]);
 
   // Logging out has to clear the context, not just call the endpoint —

frontend/src/customer/FavoritesContext.tsx

@@ -31,10 +31,13 @@ export function FavoritesProvider({ children }: Readonly<{ children: React.React
       .catch(() => setFavorites([]));
   }, []);
 
+  // Same decision as CartProvider: clearing on sign-out is synchronisation
+  // with the session rather than derived state. See #99.
   useEffect(() => {
     if (customer) {
       refresh();
     } else {
+    // eslint-disable-next-line react-hooks/set-state-in-effect
       setFavorites([]);
     }
   }, [customer, refresh]);

frontend/src/customer/Orders.tsx

@@ -115,7 +115,9 @@ export default function Orders() {
     }
   }, []);
 
-  useEffect(() => {
+  // Load-on-mount once the session resolves, with a pending flag.
+    useEffect(() => {
+  // eslint-disable-next-line react-hooks/set-state-in-effect
     if (customer) void load();
   }, [customer, load]);
 

frontend/src/customer/VerifyEmail.tsx

@@ -15,8 +15,14 @@ type Status = 'verifying' | 'verified' | 'failed';
 export default function VerifyEmail() {
   const [searchParams] = useSearchParams();
   const token = searchParams.get('token');
-  const [status, setStatus] = useState<Status>('verifying');
-  const [error, setError] = useState<string | null>(null);
+  // Derived from the URL rather than discovered in an effect. Whether the link
+  // carries a token is knowable during the first render, so routing it through
+  // an effect made this component render once as a spinner in a state that was
+  // never true — a link with no token was never "verifying". See #99.
+  const [status, setStatus] = useState<Status>(token ? 'verifying' : 'failed');
+  const [error, setError] = useState<string | null>(
+    token ? null : 'This link is missing its verification token.'
+  );
   const { refresh } = useCustomerAuth();
 
   // The backend deletes the token once it succeeds, so a second request for the
@@ -29,11 +35,8 @@ export default function VerifyEmail() {
     if (requested.current) return;
     requested.current = true;
 
-    if (!token) {
-      setStatus('failed');
-      setError('This link is missing its verification token.');
-      return;
-    }
+    // Nothing to do: the missing-token case is already the initial state.
+    if (!token) return;
 
     verifyEmail(token)
       .then(() => {

frontend/src/useCatalogue.ts

@@ -86,6 +86,8 @@ export function useCatalogue(filters: ItemFilters, onAuthRequired: () => void):
 
   useEffect(() => {
     if (awaitingAuth) {
+      // A pending flag while the session resolves, not a value already known.
+      // eslint-disable-next-line react-hooks/set-state-in-effect
       setLoading(true);
       return;
     }