fix(ui): harden navigation and queue interactions

src/utils/downloadSelection.test.ts

@@ -0,0 +1,47 @@
+import { describe, expect, it } from 'vitest';
+import { updateDownloadSelection } from './downloadSelection';
+
+const orderedIds = ['a', 'b', 'c', 'd'];
+
+describe('download selection', () => {
+  it('collapses select-all to the clicked row', () => {
+    const result = updateDownloadSelection({
+      orderedIds,
+      selectedIds: new Set(orderedIds),
+      lastSelectedId: 'd',
+      targetId: 'b',
+      extendRange: false,
+      toggle: false,
+    });
+
+    expect([...result.selectedIds]).toEqual(['b']);
+    expect(result.lastSelectedId).toBe('b');
+  });
+
+  it('toggles a row without losing the rest of the selection', () => {
+    const result = updateDownloadSelection({
+      orderedIds,
+      selectedIds: new Set(['a', 'c']),
+      lastSelectedId: 'a',
+      targetId: 'c',
+      extendRange: false,
+      toggle: true,
+    });
+
+    expect([...result.selectedIds]).toEqual(['a']);
+  });
+
+  it('extends from the existing anchor for a range selection', () => {
+    const result = updateDownloadSelection({
+      orderedIds,
+      selectedIds: new Set(['a']),
+      lastSelectedId: 'a',
+      targetId: 'c',
+      extendRange: true,
+      toggle: false,
+    });
+
+    expect([...result.selectedIds]).toEqual(['a', 'b', 'c']);
+    expect(result.lastSelectedId).toBe('a');
+  });
+});