Handle interrupted navigation transitions

tests/unit/ui-view-transition.test.ts

@@ -0,0 +1,23 @@
+import { afterEach, expect, it, vi } from 'vitest';
+import { enableViewTransitions } from '../../src/lib/ui/motion/viewTransition.js';
+
+afterEach(() => vi.unstubAllGlobals());
+
+it('allows navigation to continue when a transition is skipped or superseded', async () => {
+    let finish: () => void = () => {};
+    const finished = new Promise<void>(resolve => { finish = resolve; });
+    const complete = Promise.reject(new Error('navigation aborted'));
+    vi.stubGlobal('document', { startViewTransition: (update: () => Promise<void>) => {
+        void update().then(finish);
+        return { ready: Promise.reject(new DOMException('Skipped transition', 'AbortError')), finished };
+    } });
+    await enableViewTransitions({ complete });
+    await finished;
+});
+
+it('continues without animation when the browser cannot start a transition', async () => {
+    vi.stubGlobal('document', { startViewTransition: () => { throw new Error('Unavailable'); } });
+    await enableViewTransitions({ complete: Promise.resolve() });
+    vi.stubGlobal('document', {});
+    expect(enableViewTransitions({ complete: Promise.resolve() })).toBeUndefined();
+});