fix(ui): harden navigation and queue interactions

src/utils/downloadSelection.ts

@@ -0,0 +1,53 @@
+export interface DownloadSelectionResult {
+  selectedIds: Set<string>;
+  lastSelectedId: string | null;
+}
+
+interface DownloadSelectionOptions {
+  orderedIds: readonly string[];
+  selectedIds: ReadonlySet<string>;
+  lastSelectedId: string | null;
+  targetId: string;
+  extendRange: boolean;
+  toggle: boolean;
+}
+
+/** Apply the table's click-selection rules without depending on React state timing. */
+export const updateDownloadSelection = ({
+  orderedIds,
+  selectedIds,
+  lastSelectedId,
+  targetId,
+  extendRange,
+  toggle,
+}: DownloadSelectionOptions): DownloadSelectionResult => {
+  const targetIndex = orderedIds.indexOf(targetId);
+  if (targetIndex === -1) {
+    return { selectedIds: new Set(selectedIds), lastSelectedId };
+  }
+
+  if (extendRange && lastSelectedId) {
+    const anchorIndex = orderedIds.indexOf(lastSelectedId);
+    if (anchorIndex !== -1) {
+      const nextSelectedIds = toggle ? new Set(selectedIds) : new Set<string>();
+      const start = Math.min(anchorIndex, targetIndex);
+      const end = Math.max(anchorIndex, targetIndex);
+      for (let index = start; index <= end; index += 1) {
+        nextSelectedIds.add(orderedIds[index]);
+      }
+      return { selectedIds: nextSelectedIds, lastSelectedId };
+    }
+  }
+
+  if (toggle) {
+    const nextSelectedIds = new Set(selectedIds);
+    if (nextSelectedIds.has(targetId)) {
+      nextSelectedIds.delete(targetId);
+    } else {
+      nextSelectedIds.add(targetId);
+    }
+    return { selectedIds: nextSelectedIds, lastSelectedId: targetId };
+  }
+
+  return { selectedIds: new Set([targetId]), lastSelectedId: targetId };
+};