fix(ui): no-op when re-selecting the turn already on screen

Clicking the current-turn row in the header turn navigator while
already viewing it routed through returnToCurrent() →
viewTurn(currentTurn), which re-fetches the live report and flips the
view through `loading`. At turn 0 the only row is the live turn, so
the dropdown always fired a pointless backend round-trip and redraw.

Guard goToTurn() against re-selecting the on-screen turn
(turn === viewedTurn): just close the popover and stop. Leaving
history is unaffected — there the viewed turn differs from the target.

Closes #45

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ui/frontend/src/lib/header/turn-navigator.svelte

@@ -12,7 +12,10 @@ drawer below the 768 px breakpoint, mirroring `view-menu.svelte`.
 
 Selecting a row calls `gameState.viewTurn(N)`; the row that matches
 `currentTurn` delegates to `gameState.returnToCurrent()` so the
-"leave history" path always flows through one method.
+"leave history" path always flows through one method. Selecting the
+row already on screen (`viewedTurn`) is a pure no-op — most visibly
+at turn 0, where the sole row is the live turn — so the navigator
+never re-fetches the report just to redraw the same snapshot.
 -->
 <script lang="ts">
 	import { getContext, onMount } from "svelte";
@@ -64,6 +67,13 @@ Selecting a row calls `gameState.viewTurn(N)`; the row that matches
 	async function goToTurn(turn: number): Promise<void> {
 		open = false;
 		if (gameState === undefined) return;
+		// Re-selecting the turn already on screen changes nothing, so just
+		// close the popover. Without this guard the current-turn row routes
+		// through `returnToCurrent()` → `viewTurn(currentTurn)`, which
+		// re-fetches the live report and flips the view through `loading` —
+		// most visibly at turn 0, where the only row is the live turn.
+		// Leaving history still works: there the viewed turn differs.
+		if (turn === gameState.viewedTurn) return;
 		if (turn === gameState.currentTurn) {
 			await gameState.returnToCurrent();
 			return;

ui/frontend/tests/turn-navigator.test.ts

@@ -165,4 +165,40 @@ describe("TurnNavigator", () => {
 		expect(returnToCurrent).toHaveBeenCalledTimes(1);
 		expect(viewTurn).not.toHaveBeenCalled();
 	});
+
+	test("selecting the live row at turn 0 is a no-op (no fetch, no redraw)", async () => {
+		const store = buildStore({ currentTurn: 0, viewedTurn: 0 });
+		const viewTurn = vi
+			.spyOn(store, "viewTurn")
+			.mockResolvedValue(undefined);
+		const returnToCurrent = vi
+			.spyOn(store, "returnToCurrent")
+			.mockResolvedValue(undefined);
+		const ui = render(TurnNavigator, {
+			context: new Map([[GAME_STATE_CONTEXT_KEY, store]]),
+		});
+		await fireEvent.click(ui.getByTestId("turn-navigator-trigger"));
+		await fireEvent.click(ui.getByTestId("turn-navigator-item-0"));
+		expect(viewTurn).not.toHaveBeenCalled();
+		expect(returnToCurrent).not.toHaveBeenCalled();
+		// The click still dismisses the popover.
+		expect(ui.queryByTestId("turn-navigator-list")).not.toBeInTheDocument();
+	});
+
+	test("re-selecting the row already viewed in history is a no-op", async () => {
+		const store = buildStore({ currentTurn: 3, viewedTurn: 2 });
+		const viewTurn = vi
+			.spyOn(store, "viewTurn")
+			.mockResolvedValue(undefined);
+		const returnToCurrent = vi
+			.spyOn(store, "returnToCurrent")
+			.mockResolvedValue(undefined);
+		const ui = render(TurnNavigator, {
+			context: new Map([[GAME_STATE_CONTEXT_KEY, store]]),
+		});
+		await fireEvent.click(ui.getByTestId("turn-navigator-trigger"));
+		await fireEvent.click(ui.getByTestId("turn-navigator-item-2"));
+		expect(viewTurn).not.toHaveBeenCalled();
+		expect(returnToCurrent).not.toHaveBeenCalled();
+	});
 });