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+# Popular Requests Form Pre-fill
+
+## Goal
+
+When a user clicks a popular request on the OnlineBoard or Schedule start pages, navigate to the same start page with query params that pre-fill the search form — matching the Angular app's behavior.
+
+## Current State (Bug)
+
+Both `OnlineBoardStartPage` and `ScheduleStartPage` have empty `handlePopularRequestClick` callbacks. Clicking a popular request does nothing.
+
+Angular's behavior: sets filter state in a service before navigating to the start page. The destination filter reads from the service on init and pre-fills the form.
+
+## Design
+
+Use query params instead of a state service (aligns with React's URL-driven architecture).
+
+### Flow
+
+1. User clicks popular request
+2. `handlePopularRequestClick` reads the `PopularRequest` object
+3. Builds a URL with query params for the current start page
+4. Navigates via `useNavigate()`
+5. Start page reads query params on mount via `useSearchParams()`
+6. Passes values as initial props to the filter component
+7. User sees pre-filled form, modifies if needed, clicks "Search"
+
+### Query Param Patterns
+
+**OnlineBoard** (`/ru/onlineboard?...`):
+
+| Mode | Params |
+|------|--------|
+| FlightNumber | `tab=flight&carrier=SU&flight=0654` |
+| Departure | `tab=route&departure=LED` |
+| Arrival | `tab=route&arrival=VKO` |
+| Route | `tab=route&departure=LED&arrival=KRR` |
+
+**Schedule** (`/ru/schedule?...`):
+
+| Mode | Params |
+|------|--------|
+| Route | `departure=LED&arrival=KRR` |
+| RouteWithBack | `departure=LED&arrival=KRR&return=true` |
+
+### Files to Change
+
+1. **`src/features/online-board/components/OnlineBoardStartPage.tsx`**
+   - Implement `handlePopularRequestClick`: build query param URL, navigate
+   - Read `useSearchParams()` on mount
+   - Pass parsed values as initial props to `OnlineBoardFilter`
+
+2. **`src/features/online-board/components/OnlineBoardFilter.tsx`**
+   - No changes needed — already accepts `initialDeparture`, `initialArrival`, `initialDate`, `initialTab`, `initialFlightNumber` props
+
+3. **`src/features/schedule/components/ScheduleStartPage.tsx`**
+   - Implement `handlePopularRequestClick`: build query param URL, navigate
+   - Read `useSearchParams()` on mount
+   - Use parsed values as initial state for `departureAirport`, `arrivalAirport`, `withReturn`
+
+### What OnlineBoardFilter Already Supports
+
+```typescript
+interface OnlineBoardFilterProps {
+  initialDeparture?: string;
+  initialArrival?: string;
+  initialDate?: string;        // yyyyMMdd
+  initialTab?: "flight" | "route";
+  initialFlightNumber?: string;
+}
+```
+
+These props feed directly into `useState` initializers. No changes needed.
+
+### ScheduleStartPage Form State Changes
+
+Current (no initial support):
+```typescript
+const [departureAirport, setDepartureAirport] = useState("");
+const [arrivalAirport, setArrivalAirport] = useState("");
+const [withReturn, setWithReturn] = useState(false);
+```
+
+After (reads from query params):
+```typescript
+const [searchParams] = useSearchParams();
+const [departureAirport, setDepartureAirport] = useState(searchParams.get("departure") ?? "");
+const [arrivalAirport, setArrivalAirport] = useState(searchParams.get("arrival") ?? "");
+const [withReturn, setWithReturn] = useState(searchParams.get("return") === "true");
+```
+
+### Edge Cases
+
+- If query params are empty (normal navigation to start page): form initializes with defaults (empty strings, today's date) — no change from current behavior
+- If user clicks a popular request while the form already has values: navigating with query params replaces the page, form re-initializes with new values
+- Date is always today (matching Angular behavior which uses `new Date()`)