fix: guard arrow hotkeys when current board is not in cache

Applies CodeRabbit review feedback: only register the arrow shortcuts
when the current board is found in the boards cache (currentIndex !== -1),
otherwise the modulo arithmetic would jump to an unexpected board.

Extracts the wraparound index calculation into a pure getNextBoardIndex
helper with unit tests.

apps/nextjs/src/app/[locale]/boards/(content)/_board-navigation.spec.ts

@@ -0,0 +1,23 @@
+import { describe, expect, test } from "vitest";
+
+import { getNextBoardIndex } from "./_board-navigation";
+
+describe("get next board index", () => {
+  test.each([
+    [3, 0, 1, 1],
+    [3, 2, 1, 0],
+    [3, 2, -1, 1],
+    [3, 0, -1, 2],
+  ] as const)("moves through boards with wraparound", (boardCount, currentIndex, direction, expected) => {
+    expect(getNextBoardIndex(boardCount, currentIndex, direction)).toBe(expected);
+  });
+
+  test.each([
+    [0, 0],
+    [1, 0],
+    [3, -1],
+    [3, 3],
+  ])("does not navigate with an invalid current board", (boardCount, currentIndex) => {
+    expect(getNextBoardIndex(boardCount, currentIndex, 1)).toBeNull();
+  });
+});

apps/nextjs/src/app/[locale]/boards/(content)/_board-navigation.ts

@@ -0,0 +1,4 @@
+export const getNextBoardIndex = (boardCount: number, currentIndex: number, direction: -1 | 1) => {
+  if (boardCount <= 1 || currentIndex < 0 || currentIndex >= boardCount) return null;
+  return (currentIndex + direction + boardCount) % boardCount;
+};

apps/nextjs/src/app/[locale]/boards/(content)/_header-actions.tsx

@@ -40,6 +40,7 @@ import { CategoryEditModal } from "~/components/board/sections/category/category
 import { useDynamicSectionActions } from "~/components/board/sections/dynamic/dynamic-actions";
 import { IntegrationSelectModal } from "~/components/integration/integration-select-modal";
 import { HeaderButton } from "~/components/layout/header/button";
+import { getNextBoardIndex } from "./_board-navigation";
 
 export const BoardContentHeaderActions = ({ demoReadOnly }: { demoReadOnly: boolean }) => {
   const [isEditMode] = useEditMode();
@@ -223,9 +224,18 @@ const BoardNavigationHotkeys = () => {
   const hotkeyEntries = useMemo(() => {
     const entries: [string, () => void][] = [];
 
-    if (boards.length > 1) {
-      entries.push(["mod+shift+ArrowRight", () => navigateToBoard((currentIndex + 1) % boards.length)]);
-      entries.push(["mod+shift+ArrowLeft", () => navigateToBoard((currentIndex - 1 + boards.length) % boards.length)]);
+    // Only register the arrow shortcuts when the current board is known. If the
+    // board hasn't synced with the getAllBoards cache yet, currentIndex is -1 and
+    // the modulo arithmetic would otherwise jump to an unexpected board.
+    if (boards.length > 1 && currentIndex !== -1) {
+      entries.push(["mod+shift+ArrowRight", () => {
+        const nextIndex = getNextBoardIndex(boards.length, currentIndex, 1);
+        if (nextIndex !== null) navigateToBoard(nextIndex);
+      }]);
+      entries.push(["mod+shift+ArrowLeft", () => {
+        const nextIndex = getNextBoardIndex(boards.length, currentIndex, -1);
+        if (nextIndex !== null) navigateToBoard(nextIndex);
+      }]);
     }
 
     for (let i = 0; i < Math.min(boards.length, 9); i++) {