From 3ea29cf8b5b8165c94bf665797da5dbdf93f01ea Mon Sep 17 00:00:00 2001
From: Ilia Denisov <evilcart@gmail.com>
Date: Thu, 21 May 2026 20:29:08 +0200
Subject: [PATCH] fix(ui-calculator): keep calculator state long-lived; don't
 eject on planet click
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Move the calculator's inputs into a page-level calculatorState singleton so they survive the sidebar unmounting the tab on a tab switch (the inspector auto-opens on a planet click). ensureGame resets the design when the active game changes.

While on the calculator, a planet click no longer switches to the inspector — the calculator consumes the selection in its planet area / reach circles. Halve the reach-circle stroke width.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 .../src/lib/calculator/calc-state.svelte.ts   | 121 +++++++++++
 .../src/lib/sidebar/calculator-tab.svelte     | 191 ++++++++----------
 ui/frontend/src/map/reach-circles.ts          |   2 +-
 .../src/routes/games/[id]/+layout.svelte      |  14 +-
 ui/frontend/tests/calculator-tab.test.ts      |   9 +-
 .../tests/e2e/game-shell-inspector.spec.ts    |  36 ++++
 6 files changed, 264 insertions(+), 109 deletions(-)
 create mode 100644 ui/frontend/src/lib/calculator/calc-state.svelte.ts

diff --git a/ui/frontend/src/lib/calculator/calc-state.svelte.ts b/ui/frontend/src/lib/calculator/calc-state.svelte.ts
new file mode 100644
index 0000000..e22369e
--- /dev/null
+++ b/ui/frontend/src/lib/calculator/calc-state.svelte.ts
@@ -0,0 +1,121 @@
+// Long-lived state for the ship-class calculator. The sidebar unmounts
+// the calculator tab when another tab is active, so component-local state
+// would be lost on every tab switch (the inspector auto-opens on a planet
+// click, for instance). The calculator is a long-lived planning tool, so
+// its inputs live here — a page-level singleton that survives tab
+// unmount/remount — and the component renders this store rather than its
+// own `$state`.
+//
+// `ensureGame` resets the design when the active game changes so a draft
+// from a previous game does not leak across games. `reset` is for tests,
+// which share the module instance across cases.
+
+import type { LoadMode, LockableOutputId } from "./calc-model";
+
+interface Blocks {
+	drive: number;
+	armament: number;
+	weapons: number;
+	shields: number;
+	cargo: number;
+}
+interface Tech {
+	drive: number;
+	weapons: number;
+	shields: number;
+	cargo: number;
+}
+type Mode = "ship" | "modernization";
+
+function freshBlocks(): Blocks {
+	return { drive: 0, armament: 0, weapons: 0, shields: 0, cargo: 0 };
+}
+function freshTech(): Tech {
+	return { drive: 0, weapons: 0, shields: 0, cargo: 0 };
+}
+
+class CalculatorState {
+	gameId = $state<string | null>(null);
+	mode = $state<Mode>("ship");
+	name = $state("");
+	blocks = $state<Blocks>(freshBlocks());
+	techValues = $state<Tech>(freshTech());
+	techOverridden = $state<Record<keyof Tech, boolean>>({
+		drive: false,
+		weapons: false,
+		shields: false,
+		cargo: false,
+	});
+	targetTech = $state<Tech>(freshTech());
+	targetSeeded = $state(false);
+	loadMode = $state<LoadMode>("full");
+	customLoad = $state(0);
+	lock = $state<LockableOutputId | null>(null);
+	lockValue = $state(0);
+	matOverridden = $state(false);
+	matValue = $state(0);
+	loadedExisting = $state<string | null>(null);
+	// The last calculatorLoadRequest token this state has applied. Held
+	// here (not in the component) so a tab-switch remount does not
+	// re-apply the previous load request and clobber the kept design.
+	handledLoadToken = $state(0);
+
+	/** Clears the design back to a blank new-class form. */
+	resetDesign(): void {
+		this.blocks = freshBlocks();
+		this.name = "";
+		this.loadedExisting = null;
+		this.lock = null;
+	}
+
+	/** Full reset to defaults; used by tests sharing the singleton. */
+	reset(): void {
+		this.gameId = null;
+		this.mode = "ship";
+		this.resetDesign();
+		this.techValues = freshTech();
+		this.techOverridden = {
+			drive: false,
+			weapons: false,
+			shields: false,
+			cargo: false,
+		};
+		this.targetTech = freshTech();
+		this.targetSeeded = false;
+		this.loadMode = "full";
+		this.customLoad = 0;
+		this.lockValue = 0;
+		this.matOverridden = false;
+		this.matValue = 0;
+		this.handledLoadToken = 0;
+	}
+
+	/**
+	 * Resets the per-game design when the active game changes, so a draft
+	 * from one game does not surface in another. A no-op while the game is
+	 * unchanged, which is what makes the design survive tab switches.
+	 * `handledLoadToken` is intentionally preserved across games.
+	 */
+	ensureGame(gameId: string): void {
+		if (this.gameId === gameId) return;
+		this.gameId = gameId;
+		this.mode = "ship";
+		this.resetDesign();
+		this.techValues = freshTech();
+		this.techOverridden = {
+			drive: false,
+			weapons: false,
+			shields: false,
+			cargo: false,
+		};
+		this.targetTech = freshTech();
+		this.targetSeeded = false;
+		this.loadMode = "full";
+		this.customLoad = 0;
+		this.lockValue = 0;
+		this.matOverridden = false;
+		this.matValue = 0;
+	}
+}
+
+export const calculatorState = new CalculatorState();
diff --git a/ui/frontend/src/lib/sidebar/calculator-tab.svelte b/ui/frontend/src/lib/sidebar/calculator-tab.svelte
index c70a651..0b64afc 100644
--- a/ui/frontend/src/lib/sidebar/calculator-tab.svelte
+++ b/ui/frontend/src/lib/sidebar/calculator-tab.svelte
@@ -6,14 +6,16 @@ into one sidebar tool, and adds single-target goal-seek — the player pins
 one result and the model back-solves the single input it claims (see
 `lib/calculator/calc-model.ts`). A second mode reuses the design area to
 price ship-class modernization. All math comes from `pkg/calc` through
-the `Core` WASM bridge; this component only holds input state and renders.
+the `Core` WASM bridge; this component only renders + orchestrates.
 
-State is component-local: the sidebar keeps this tab mounted while the
-player navigates between active views, so inputs persist across view
-switches per the global state-preservation rule.
+Input state lives in the long-lived `calculatorState` singleton, not in
+the component, so it survives the sidebar unmounting this tab on a tab
+switch (the inspector auto-opens on a planet click) — the calculator is a
+long-lived planning tool. `ensureGame` resets it when the game changes.
 -->
 <script lang="ts">
 	import { getContext } from "svelte";
+	import { page } from "$app/state";
 
 	import { i18n, type TranslationKey } from "$lib/i18n/index.svelte";
 	import {
@@ -40,13 +42,14 @@ switches per the global state-preservation rule.
 		type LockableOutputId,
 		type LoadMode,
 	} from "$lib/calculator/calc-model";
-
-	const LOAD_MODES: LoadMode[] = ["empty", "full", "custom"];
 	import ShipDesignArea, {
 		type TechKey,
 	} from "$lib/calculator/ship-design-area.svelte";
 	import { reachStore } from "$lib/calculator/reach.svelte";
 	import { calculatorLoadRequest } from "$lib/calculator/load-request.svelte";
+	import { calculatorState } from "$lib/calculator/calc-state.svelte";
+
+	const LOAD_MODES: LoadMode[] = ["empty", "full", "custom"];
 
 	const rendered = getContext<RenderedReportSource | undefined>(
 		RENDERED_REPORT_CONTEXT_KEY,
@@ -59,26 +62,13 @@ switches per the global state-preservation rule.
 		SELECTION_CONTEXT_KEY,
 	);
 
-	type Mode = "ship" | "modernization";
-	let mode = $state<Mode>("ship");
-
-	let name = $state("");
-	let blocks = $state({ drive: 0, armament: 0, weapons: 0, shields: 0, cargo: 0 });
-	let techValues = $state({ drive: 0, weapons: 0, shields: 0, cargo: 0 });
-	let techOverridden = $state<Record<TechKey, boolean>>({
-		drive: false,
-		weapons: false,
-		shields: false,
-		cargo: false,
+	// The long-lived input state (survives tab unmount/remount).
+	const cs = calculatorState;
+	// Reset the design when the active game changes; a no-op otherwise, so
+	// the design persists across tab switches within a game.
+	$effect(() => {
+		cs.ensureGame(page.params.id ?? "");
 	});
-	let targetTech = $state({ drive: 0, weapons: 0, shields: 0, cargo: 0 });
-	let loadMode = $state<LoadMode>("full");
-	let customLoad = $state(0);
-	let lock = $state<LockableOutputId | null>(null);
-	let lockValue = $state(0);
-	let matOverridden = $state(false);
-	let matValue = $state(0);
-	let loadedExisting = $state<string | null>(null);
 
 	const core = $derived(coreHandle?.core ?? null);
 	const report = $derived(rendered?.report ?? null);
@@ -101,36 +91,35 @@ switches per the global state-preservation rule.
 	// changes, so the calculator reflects the right turn's tech.
 	$effect(() => {
 		for (const k of techKeys) {
-			if (!techOverridden[k]) techValues[k] = playerTech[k];
+			if (!cs.techOverridden[k]) cs.techValues[k] = playerTech[k];
 		}
 	});
 	// Seed the modernization target with the player's current tech once
 	// the report has loaded; afterwards it is the player's to edit.
-	let targetSeeded = false;
 	$effect(() => {
-		if (targetSeeded) return;
+		if (cs.targetSeeded) return;
 		if (
 			playerTech.drive ||
 			playerTech.weapons ||
 			playerTech.shields ||
 			playerTech.cargo
 		) {
-			targetTech = { ...playerTech };
-			targetSeeded = true;
+			cs.targetTech = { ...playerTech };
+			cs.targetSeeded = true;
 		}
 	});
 
 	const result = $derived(
 		computeCalculator(
 			{
-				blocks,
-				driveTech: techValues.drive,
-				weaponsTech: techValues.weapons,
-				shieldsTech: techValues.shields,
-				cargoTech: techValues.cargo,
-				loadMode,
-				customLoad,
-				lock: lock === null ? null : { output: lock, value: lockValue },
+				blocks: cs.blocks,
+				driveTech: cs.techValues.drive,
+				weaponsTech: cs.techValues.weapons,
+				shieldsTech: cs.techValues.shields,
+				cargoTech: cs.techValues.cargo,
+				loadMode: cs.loadMode,
+				customLoad: cs.customLoad,
+				lock: cs.lock === null ? null : { output: cs.lock, value: cs.lockValue },
 			},
 			core,
 		),
@@ -146,8 +135,8 @@ switches per the global state-preservation rule.
 	});
 
 	$effect(() => {
-		if (!matOverridden) {
-			matValue = selectedPlanet?.materialsStockpile ?? 0;
+		if (!cs.matOverridden) {
+			cs.matValue = selectedPlanet?.materialsStockpile ?? 0;
 		}
 	});
 
@@ -159,7 +148,7 @@ switches per the global state-preservation rule.
 			{
 				shipMass: emptyMass,
 				freeIndustry: selectedPlanet.freeIndustry ?? 0,
-				material: matValue,
+				material: cs.matValue,
 				resources: selectedPlanet.resources ?? 0,
 			},
 			core,
@@ -171,7 +160,7 @@ switches per the global state-preservation rule.
 	// own planet is selected, or the calculator is in modernization mode.
 	$effect(() => {
 		const out = result.outputs;
-		if (mode === "ship" && selectedPlanet !== null && out !== null) {
+		if (cs.mode === "ship" && selectedPlanet !== null && out !== null) {
 			reachStore.set(
 				{ x: selectedPlanet.x, y: selectedPlanet.y },
 				out.speedLoaded,
@@ -202,15 +191,15 @@ switches per the global state-preservation rule.
 	};
 
 	const nameValidation = $derived(
-		validateShipClass({ name, ...result.blocks }, { existingNames }),
+		validateShipClass({ name: cs.name, ...result.blocks }, { existingNames }),
 	);
 	const createMessage = $derived(
 		nameValidation.ok ? "" : i18n.t(nameInvalidKeyMap[nameValidation.reason]),
 	);
 	const canCreate = $derived(nameValidation.ok && draft !== undefined);
 	const canDelete = $derived(
-		loadedExisting !== null &&
-			existingNames.includes(loadedExisting) &&
+		cs.loadedExisting !== null &&
+			existingNames.includes(cs.loadedExisting) &&
 			draft !== undefined,
 	);
 
@@ -218,21 +207,21 @@ switches per the global state-preservation rule.
 	const modernCosts = $derived.by(() => {
 		if (core === null) return null;
 		const weaponsMass = core.weaponsBlockMass({
-			weapons: blocks.weapons,
-			armament: blocks.armament,
+			weapons: cs.blocks.weapons,
+			armament: cs.blocks.armament,
 		});
 		const rows: { key: TechKey; mass: number }[] = [
-			{ key: "drive", mass: blocks.drive },
+			{ key: "drive", mass: cs.blocks.drive },
 			{ key: "weapons", mass: weaponsMass ?? 0 },
-			{ key: "shields", mass: blocks.shields },
-			{ key: "cargo", mass: blocks.cargo },
+			{ key: "shields", mass: cs.blocks.shields },
+			{ key: "cargo", mass: cs.blocks.cargo },
 		];
 		const perBlock = rows.map((r) => ({
 			key: r.key,
 			cost: core.blockUpgradeCost({
 				blockMass: r.mass,
-				currentTech: techValues[r.key],
-				targetTech: targetTech[r.key],
+				currentTech: cs.techValues[r.key],
+				targetTech: cs.targetTech[r.key],
 			}),
 		}));
 		const total = perBlock.reduce((sum, r) => sum + r.cost, 0);
@@ -247,59 +236,51 @@ switches per the global state-preservation rule.
 	}
 
 	function onTechInput(key: TechKey): void {
-		techOverridden[key] = true;
+		cs.techOverridden[key] = true;
 	}
 	function onResetTech(key: TechKey): void {
-		techOverridden[key] = false;
+		cs.techOverridden[key] = false;
 	}
 	function onMatInput(): void {
-		matOverridden = true;
+		cs.matOverridden = true;
 	}
 	function resetMat(): void {
-		matOverridden = false;
+		cs.matOverridden = false;
 	}
 
 	function lockOutput(output: LockableOutputId): void {
-		if (lock !== null) return;
-		lockValue = result.outputs?.[output] ?? 0;
-		lock = output;
+		if (cs.lock !== null) return;
+		cs.lockValue = result.outputs?.[output] ?? 0;
+		cs.lock = output;
 	}
 	function unlock(): void {
-		lock = null;
+		cs.lock = null;
 	}
 
 	function loadExisting(clsName: string): void {
 		const cls = localShipClass.find((c) => c.name === clsName);
 		if (cls === undefined) return;
-		blocks = {
+		cs.blocks = {
 			drive: cls.drive,
 			armament: cls.armament,
 			weapons: cls.weapons,
 			shields: cls.shields,
 			cargo: cls.cargo,
 		};
-		name = cls.name;
-		loadedExisting = cls.name;
-		lock = null;
-	}
-
-	function resetToNew(): void {
-		blocks = { drive: 0, armament: 0, weapons: 0, shields: 0, cargo: 0 };
-		name = "";
-		loadedExisting = null;
-		lock = null;
+		cs.name = cls.name;
+		cs.loadedExisting = cls.name;
+		cs.lock = null;
 	}
 
 	// React to the ship-classes table / bottom-tabs asking to load a
 	// class (or start a fresh design) into the calculator. The layout
 	// flips the sidebar to this tab in parallel.
-	let lastLoadToken = 0;
 	$effect(() => {
 		const token = calculatorLoadRequest.token;
-		if (token === lastLoadToken) return;
-		lastLoadToken = token;
-		mode = "ship";
-		if (calculatorLoadRequest.name === null) resetToNew();
+		if (token === cs.handledLoadToken) return;
+		cs.handledLoadToken = token;
+		cs.mode = "ship";
+		if (calculatorLoadRequest.name === null) cs.resetDesign();
 		else loadExisting(calculatorLoadRequest.name);
 	});
 
@@ -320,17 +301,17 @@ switches per the global state-preservation rule.
 			shields: created.shields,
 			cargo: created.cargo,
 		});
-		loadedExisting = created.name;
+		cs.loadedExisting = created.name;
 	}
 
 	async function deleteClass(): Promise<void> {
-		if (loadedExisting === null || draft === undefined) return;
+		if (cs.loadedExisting === null || draft === undefined) return;
 		await draft.add({
 			kind: "removeShipClass",
 			id: crypto.randomUUID(),
-			name: loadedExisting,
+			name: cs.loadedExisting,
 		});
-		loadedExisting = null;
+		cs.loadedExisting = null;
 	}
 
 	const LOCK_LABELS: Record<LockableOutputId, string> = $derived({
@@ -344,12 +325,12 @@ switches per the global state-preservation rule.
 </script>
 
 {#snippet lockable(output: LockableOutputId, value: number | undefined)}
-	{#if lock === output}
+	{#if cs.lock === output}
 		<span class="cell locked" class:infeasible={!result.lockFeasible}>
 			<input
 				type="number"
 				step="0.001"
-				bind:value={lockValue}
+				bind:value={cs.lockValue}
 				data-testid={`calculator-locked-${output}`}
 				title={result.lockFeasible ? "" : i18n.t("game.calculator.lock.infeasible")}
 			/>
@@ -370,8 +351,8 @@ switches per the global state-preservation rule.
 			<button
 				type="button"
 				class="lock"
-				disabled={lock !== null || value === undefined}
-				title={lock !== null
+				disabled={cs.lock !== null || value === undefined}
+				title={cs.lock !== null
 					? i18n.t("game.calculator.lock.max")
 					: `${LOCK_LABELS[output]}`}
 				aria-label={LOCK_LABELS[output]}
@@ -388,17 +369,17 @@ switches per the global state-preservation rule.
 	<div class="modes" role="tablist">
 		<button
 			type="button"
-			class:active={mode === "ship"}
+			class:active={cs.mode === "ship"}
 			data-testid="calculator-mode-ship"
-			onclick={() => (mode = "ship")}
+			onclick={() => (cs.mode = "ship")}
 		>
 			{i18n.t("game.calculator.mode.ship")}
 		</button>
 		<button
 			type="button"
-			class:active={mode === "modernization"}
+			class:active={cs.mode === "modernization"}
 			data-testid="calculator-mode-modernization"
-			onclick={() => (mode = "modernization")}
+			onclick={() => (cs.mode = "modernization")}
 		>
 			{i18n.t("game.calculator.mode.modernization")}
 		</button>
@@ -411,9 +392,9 @@ switches per the global state-preservation rule.
 			list="calculator-existing-classes"
 			placeholder={i18n.t("game.calculator.name.placeholder")}
 			maxlength="30"
-			bind:value={name}
-			oninput={() => (loadedExisting = null)}
-			onchange={() => loadExisting(name)}
+			bind:value={cs.name}
+			oninput={() => (cs.loadedExisting = null)}
+			onchange={() => loadExisting(cs.name)}
 			aria-invalid={nameValidation.ok ? "false" : "true"}
 			data-testid="calculator-name"
 		/>
@@ -422,7 +403,7 @@ switches per the global state-preservation rule.
 				<option value={cls.name}></option>
 			{/each}
 		</datalist>
-		{#if mode === "ship"}
+		{#if cs.mode === "ship"}
 			<button
 				type="button"
 				class="create"
@@ -435,49 +416,49 @@ switches per the global state-preservation rule.
 			</button>
 		{/if}
 	</div>
-	{#if mode === "ship" && canDelete}
+	{#if cs.mode === "ship" && canDelete}
 		<button
 			type="button"
 			class="delete"
 			data-testid="calculator-delete"
 			onclick={() => void deleteClass()}
 		>
-			{i18n.t("game.calculator.action.delete")} {loadedExisting}
+			{i18n.t("game.calculator.action.delete")} {cs.loadedExisting}
 		</button>
 	{/if}
 
 	<ShipDesignArea
-		bind:blocks
+		bind:blocks={cs.blocks}
 		resolved={result.blocks}
-		bind:techs={techValues}
-		{techOverridden}
+		bind:techs={cs.techValues}
+		techOverridden={cs.techOverridden}
 		computedInput={result.computedInput}
 		{onTechInput}
 		{onResetTech}
 	/>
 
-	{#if mode === "ship"}
+	{#if cs.mode === "ship"}
 		<div class="load">
 			<span class="label">{i18n.t("game.calculator.load.label")}</span>
 			<div class="seg" role="group">
 				{#each LOAD_MODES as m (m)}
 					<button
 						type="button"
-						class:active={loadMode === m}
+						class:active={cs.loadMode === m}
 						data-testid={`calculator-load-${m}`}
-						onclick={() => (loadMode = m)}
+						onclick={() => (cs.loadMode = m)}
 					>
 						{i18n.t(`game.calculator.load.${m}` as TranslationKey)}
 					</button>
 				{/each}
 			</div>
-			{#if loadMode === "custom"}
+			{#if cs.loadMode === "custom"}
 				<input
 					type="number"
 					step="0.01"
 					min="0"
 					class="custom-load"
-					bind:value={customLoad}
+					bind:value={cs.customLoad}
 					data-testid="calculator-custom-load"
 				/>
 			{/if}
@@ -548,11 +529,11 @@ switches per the global state-preservation rule.
 							type="number"
 							step="0.01"
 							min="0"
-							bind:value={matValue}
+							bind:value={cs.matValue}
 							oninput={onMatInput}
 							data-testid="calculator-planet-mat"
 						/>
-						{#if matOverridden}
+						{#if cs.matOverridden}
 							<button
 								type="button"
 								class="lock active"
@@ -598,7 +579,7 @@ switches per the global state-preservation rule.
 							type="number"
 							step="0.001"
 							min="0"
-							bind:value={targetTech[row.key]}
+							bind:value={cs.targetTech[row.key]}
 							data-testid={`calculator-target-${row.key}`}
 						/>
 					</span>
diff --git a/ui/frontend/src/map/reach-circles.ts b/ui/frontend/src/map/reach-circles.ts
index 670d679..d221ce7 100644
--- a/ui/frontend/src/map/reach-circles.ts
+++ b/ui/frontend/src/map/reach-circles.ts
@@ -73,7 +73,7 @@ export function computeReachCircles(
 			style: {
 				strokeColor: REACH_CIRCLE_COLOR,
 				strokeAlpha: 0.55 - (turn - 1) * 0.12,
-				strokeWidthPx: 1,
+				strokeWidthPx: 0.5,
 			},
 		});
 	}
diff --git a/ui/frontend/src/routes/games/[id]/+layout.svelte b/ui/frontend/src/routes/games/[id]/+layout.svelte
index c647b9a..5787600 100644
--- a/ui/frontend/src/routes/games/[id]/+layout.svelte
+++ b/ui/frontend/src/routes/games/[id]/+layout.svelte
@@ -43,7 +43,7 @@ the next game's snapshot — and the next game's selection — start
 fresh.
 -->
 <script lang="ts">
-	import { onDestroy, onMount, setContext } from "svelte";
+	import { onDestroy, onMount, setContext, untrack } from "svelte";
 	import { goto } from "$app/navigation";
 	import { page } from "$app/state";
 	import Header from "$lib/header/header.svelte";
@@ -223,7 +223,17 @@ fresh.
 	$effect(() => {
 		const sel = selection.selected;
 		if (sel === null) return;
-		activeTab = "inspector";
+		// Stay in the calculator when a planet is picked: the calculator
+		// consumes the selection in its planet area + reach circles, and
+		// it is a long-lived workspace the user should not be ejected
+		// from. `activeTab` is read untracked so a manual tab switch does
+		// not re-fire this effect. Any other case (including a ship-group
+		// selection, which the calculator does not use) reveals the
+		// inspector as before.
+		const tab = untrack(() => activeTab);
+		if (!(tab === "calculator" && sel.kind === "planet")) {
+			activeTab = "inspector";
+		}
 		sidebarOpen = true;
 	});
 
diff --git a/ui/frontend/tests/calculator-tab.test.ts b/ui/frontend/tests/calculator-tab.test.ts
index 4251705..91efa83 100644
--- a/ui/frontend/tests/calculator-tab.test.ts
+++ b/ui/frontend/tests/calculator-tab.test.ts
@@ -7,10 +7,15 @@
 import "@testing-library/jest-dom/vitest";
 import "fake-indexeddb/auto";
 import { fireEvent, render } from "@testing-library/svelte";
-import { afterEach, beforeEach, describe, expect, test } from "vitest";
+import { afterEach, beforeEach, describe, expect, test, vi } from "vitest";
+
+// The calculator reads `page.params.id` to scope its long-lived state to
+// the active game; stub a stable id so the component test has a router.
+vi.mock("$app/state", () => ({ page: { params: { id: "calc-test-game" } } }));
 
 import { i18n } from "../src/lib/i18n/index.svelte";
 import CalculatorTab from "../src/lib/sidebar/calculator-tab.svelte";
+import { calculatorState } from "../src/lib/calculator/calc-state.svelte";
 import { CORE_CONTEXT_KEY, CoreHolder } from "../src/lib/core-context.svelte";
 import {
 	ORDER_DRAFT_CONTEXT_KEY,
@@ -106,6 +111,8 @@ beforeEach(async () => {
 	draft = new OrderDraftStore();
 	await draft.init({ cache: new IDBCache(db), gameId: GAME_ID });
 	i18n.resetForTests("en");
+	// The calculator state is a module singleton shared across cases.
+	calculatorState.reset();
 });
 
 afterEach(async () => {
diff --git a/ui/frontend/tests/e2e/game-shell-inspector.spec.ts b/ui/frontend/tests/e2e/game-shell-inspector.spec.ts
index d1ddf0e..f971e04 100644
--- a/ui/frontend/tests/e2e/game-shell-inspector.spec.ts
+++ b/ui/frontend/tests/e2e/game-shell-inspector.spec.ts
@@ -278,3 +278,39 @@ test("calculator draws reach circles for the selected planet", async ({
 	await calc.getByTestId("calculator-mode-modernization").click();
 	await expect.poll(() => countReachCircles(page)).toBe(0);
 });
+
+test("calculator stays put on a planet click and keeps state across tab switches", async ({
+	page,
+}, testInfo) => {
+	test.skip(
+		testInfo.project.name.startsWith("chromium-mobile"),
+		"calculator is a desktop-sidebar flow",
+	);
+	await setupShell(page);
+
+	// Open the calculator and enter a design.
+	await page.getByTestId("sidebar-tab-calculator").click();
+	await page.getByTestId("calculator-block-drive").fill("10");
+
+	// Clicking a planet must NOT eject us to the inspector; it feeds the
+	// calculator's planet area instead, and the design is untouched.
+	await clickCanvasCentre(page);
+	await expect(page.getByTestId("sidebar")).toHaveAttribute(
+		"data-active-tab",
+		"calculator",
+	);
+	await expect(page.getByTestId("calculator-planet-name")).toContainText(
+		"Galactica",
+	);
+	await expect(page.getByTestId("calculator-block-drive")).toHaveValue("10");
+
+	// Switching to the inspector and back keeps the design (long-lived
+	// tool state survives the tab unmount/remount).
+	await page.getByTestId("sidebar-tab-inspector").click();
+	await expect(page.getByTestId("sidebar")).toHaveAttribute(
+		"data-active-tab",
+		"inspector",
+	);
+	await page.getByTestId("sidebar-tab-calculator").click();
+	await expect(page.getByTestId("calculator-block-drive")).toHaveValue("10");
+});