2 files changed, 104 insertions, 0 deletions
diff --git a/src/features/views/logic/panels.test.ts b/src/features/views/logic/panels.test.ts
new file mode 100644
index 0000000..edd7d9e
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+++ b/src/features/views/logic/panels.test.ts
@@ -0,0 +1,55 @@
+import { describe, expect, it } from "vitest";
+import { addPanel, initialPanels, removePanel, selectKind } from "./panels";
+
+describe("view panels reducer", () => {
+	it("seeds one empty panel by default", () => {
+		const s = initialPanels();
+		expect(s.panels).toHaveLength(1);
+		expect(s.panels[0]?.kind).toBeNull();
+	});
+
+	it("seeds a panel per provided kind, in order, with unique ids", () => {
+		const s = initialPanels(["surfaces", null]);
+		expect(s.panels.map((p) => p.kind)).toEqual(["surfaces", null]);
+		expect(new Set(s.panels.map((p) => p.id)).size).toBe(2);
+	});
+
+	it("addPanel appends an empty panel with a fresh id", () => {
+		const seed = initialPanels(["surfaces"]);
+		const s = addPanel(seed);
+		expect(s.panels).toHaveLength(2);
+		expect(s.panels[1]?.kind).toBeNull();
+		expect(s.panels[1]?.id).not.toBe(s.panels[0]?.id);
+	});
+
+	it("addPanel can seed a kind", () => {
+		const s = addPanel(initialPanels([null]), "surfaces");
+		expect(s.panels[1]?.kind).toBe("surfaces");
+	});
+
+	it("removePanel drops the matching id only", () => {
+		const seed = initialPanels(["surfaces", null]);
+		const firstId = seed.panels[0]?.id ?? -1;
+		const s = removePanel(seed, firstId);
+		expect(s.panels).toHaveLength(1);
+		expect(s.panels[0]?.kind).toBeNull();
+	});
+
+	it("selectKind updates only the targeted panel", () => {
+		const seed = initialPanels([null, null]);
+		const targetId = seed.panels[1]?.id ?? -1;
+		const s = selectKind(seed, targetId, "surfaces");
+		expect(s.panels[0]?.kind).toBeNull();
+		expect(s.panels[1]?.kind).toBe("surfaces");
+	});
+
+	it("is pure — never mutates the input state", () => {
+		const seed = initialPanels(["surfaces"]);
+		const snapshot = JSON.stringify(seed);
+		const id = seed.panels[0]?.id ?? -1;
+		addPanel(seed);
+		removePanel(seed, id);
+		selectKind(seed, id, null);
+		expect(JSON.stringify(seed)).toBe(snapshot);
+	});
+});
diff --git a/src/features/views/logic/panels.ts b/src/features/views/logic/panels.ts
new file mode 100644
index 0000000..38c28fb
--- /dev/null
+++ b/src/features/views/logic/panels.ts
@@ -0,0 +1,49 @@
+/**
+ * Pure reducer for the view sidebar's panel stack.
+ *
+ * A "view" is the RESERVED Dispatch sidebar affordance (see GLOSSARY): the user
+ * stacks panels, each showing one view KIND chosen from a dropdown, and adds
+ * more with a `+` button. This module is the pure model — zero DOM, zero Svelte.
+ * The component (`ViewSidebar.svelte`) is a thin runes wrapper over it.
+ *
+ * `id` is a per-session stable key for `{#each}` only; it is never persisted.
+ */
+
+export interface ViewPanel {
+	readonly id: number;
+	/** Selected view-kind id, or `null` while the panel still reads "Select a view". */
+	readonly kind: string | null;
+}
+
+export interface PanelsState {
+	readonly panels: readonly ViewPanel[];
+	readonly nextId: number;
+}
+
+/**
+ * Seed state. Each entry becomes one panel in order; pass `["surfaces"]` to open
+ * a single preset panel, or `[null]` for one empty "Select a view" panel.
+ */
+export function initialPanels(kinds: readonly (string | null)[] = [null]): PanelsState {
+	let nextId = 0;
+	const panels = kinds.map((kind) => ({ id: nextId++, kind }));
+	return { panels, nextId };
+}
+
+export function addPanel(state: PanelsState, kind: string | null = null): PanelsState {
+	return {
+		panels: [...state.panels, { id: state.nextId, kind }],
+		nextId: state.nextId + 1,
+	};
+}
+
+export function removePanel(state: PanelsState, id: number): PanelsState {
+	return { ...state, panels: state.panels.filter((p) => p.id !== id) };
+}
+
+export function selectKind(state: PanelsState, id: number, kind: string | null): PanelsState {
+	return {
+		...state,
+		panels: state.panels.map((p) => (p.id === id ? { ...p, kind } : p)),
+	};
+}