refactor(web): TopBar closeTab lands through tabLanding, the one copy of the Q3 rule (TASK-3280) (#1642)

web/src/lib/components/layout/TopBar.svelte

@@ -17,6 +17,7 @@
 	import Menu from '$lib/components/common/Menu.svelte';
 	import MenuItem from '$lib/components/common/MenuItem.svelte';
 	import { workspaceRestoreTarget } from '$lib/utils/workspace-route';
+	import { tabLanding } from '$lib/utils/tabLanding';
 
 	let { mobile = false }: { mobile?: boolean } = $props();
 
@@ -105,16 +106,14 @@
 		if (tab.ephemeral) void tabsStore.pin(tab.slug).catch(() => {});
 	}
 
-	// Closing a tab (PLAN-3002 Q2, Q3). Only closing the ACTIVE tab moves you:
-	// to its left neighbour, or, when it was the first tab, to the tab that
-	// becomes first (its right neighbour; lead ruling on TASK-3274), at that
-	// tab's last route. Closing the last tab lands on /console with "+"
-	// highlighted as the way back in.
+	// Closing a tab (PLAN-3002 Q2, Q3). Only closing the ACTIVE tab moves you,
+	// to where tabLanding says (the one copy of the Q3 rule, shared with the
+	// lost-workspace path; TASK-3280), at that tab's last route. Closing the
+	// last tab lands on /console with "+" highlighted as the way back in.
 	async function closeTab(e: MouseEvent, tab: WorkspaceTab) {
 		e.preventDefault();
 		e.stopPropagation();
-		const before = tabsStore.tabs.map((t) => t.slug);
-		const idx = before.indexOf(tab.slug);
+		const before = tabsStore.tabs.slice();
 		const wasActive = tab.slug === currentSlug;
 		try {
 			await tabsStore.close(tab.slug);
@@ -122,8 +121,8 @@
 			return;
 		}
 		if (!wasActive) return;
-		const after = tabsStore.tabs;
-		if (after.length === 0) {
+		const landing = tabLanding(before, tab.slug, tabsStore.tabs);
+		if (!landing) {
 			// Set once the navigation has landed: /console has no TopBar, so
 			// the console page shows it on its own create button and clears it
 			// when it unmounts.
@@ -131,8 +130,6 @@
 			uiStore.highlightAddWorkspace();
 			return;
 		}
-		const neighbour = idx > 0 ? before[idx - 1] : before[idx + 1];
-		const landing = after.find((t) => t.slug === neighbour) ?? after[0];
 		goto(workspaceRestoreTarget(landing));
 	}
 

web/src/lib/components/layout/TopBar.svelte.test.ts

@@ -194,6 +194,18 @@ describe('TopBar tab bar: closing (PLAN-3002 Q2, Q3)', () => {
 		expect(mocks.goto).toHaveBeenCalledWith('/u/alpha/docs');
 	});
 
+	// The leg above closes the SECOND tab, whose left neighbour is also the
+	// first tab, so it cannot tell "left neighbour" from "first remaining"
+	// (TASK-3280: a closeTab that read the open set after the close passed it).
+	it('closing an active tab further right lands on its left neighbour, not on the first tab', async () => {
+		mocks.current = { slug: 'gamma', name: 'Gamma', owner_username: 'u' };
+		await mountWith([ALPHA, BETA, GAMMA]);
+		mocks.tabs.close.mockResolvedValueOnce(answer([ALPHA, BETA]));
+		closeBtn('gamma').click();
+		await settle();
+		expect(mocks.goto).toHaveBeenCalledWith('/u/beta');
+	});
+
 	it('closing the active FIRST tab lands on the tab that becomes first (lead ruling on Q3)', async () => {
 		mocks.current = { slug: 'alpha', name: 'Alpha', owner_username: 'u' };
 		await mountWith([ALPHA, BETA, GAMMA]);

web/src/lib/utils/tabLanding.test.ts

@@ -37,6 +37,22 @@ describe('tabLanding (PLAN-3002 Q3)', () => {
 		expect(tabLanding(before, 'a', tabs('c'))?.slug).toBe('c');
 	});
 
+	// The two legs below give `after` a shape that is NOT `before` minus the
+	// gone tab, which is the only case where these rules differ from their
+	// look-alikes (TASK-3280: "first remaining" vs "last remaining", and
+	// "right neighbour" vs "first remaining").
+	it('the fallback is the FIRST remaining tab, not any remaining tab', () => {
+		const before = tabs('a', 'b', 'c', 'd');
+		// d went, and so did its neighbour c: two tabs remain.
+		expect(tabLanding(before, 'd', tabs('a', 'b'))?.slug).toBe('a');
+	});
+
+	it('when the first tab goes, its right neighbour wins even if it is no longer first', () => {
+		// The refetch also opened x at the front of the bar.
+		const before = tabs('a', 'b', 'c');
+		expect(tabLanding(before, 'a', tabs('x', 'b', 'c'))?.slug).toBe('b');
+	});
+
 	it('lands on the first tab when the gone workspace was never in the bar', () => {
 		expect(tabLanding(tabs('a', 'b'), 'zz', tabs('a', 'b'))?.slug).toBe('a');
 	});