diff --git a/src/ui/App.tsx b/src/ui/App.tsx
index 56b71523..c3e8ea00 100644
--- a/src/ui/App.tsx
+++ b/src/ui/App.tsx
@@ -23,13 +23,12 @@ import { useHunkSessionBridge } from "./hooks/useHunkSessionBridge";
 import { useMenuController } from "./hooks/useMenuController";
 import { useReviewController } from "./hooks/useReviewController";
 import { buildAppMenus } from "./lib/appMenus";
+import type { FocusArea } from "./lib/focus";
 import { fileRowId } from "./lib/ids";
 import { resolveResponsiveLayout } from "./lib/responsive";
 import { resizeSidebarWidth } from "./lib/sidebar";
 import { resolveTheme, THEMES } from "./themes";
 
-type FocusArea = "files" | "filter";
-
 const FAST_CODE_HORIZONTAL_SCROLL_COLUMNS = 8;
 
 const LazyHelpDialog = lazy(async () => ({
@@ -113,7 +112,7 @@ export function App({
   const [sidebarVisible, setSidebarVisible] = useState(() => !pagerMode);
   const [forceSidebarOpen, setForceSidebarOpen] = useState(false);
   const [showHelp, setShowHelp] = useState(false);
-  const [focusArea, setFocusArea] = useState<FocusArea>("files");
+  const [focusArea, setFocusArea] = useState<FocusArea>("diff");
   const [sidebarWidth, setSidebarWidth] = useState(34);
   const [resizeDragOriginX, setResizeDragOriginX] = useState<number | null>(null);
   const [resizeStartWidth, setResizeStartWidth] = useState<number | null>(null);
@@ -455,9 +454,11 @@ export function App({
     setFocusArea("filter");
   }, []);
 
-  /** Toggle keyboard focus between the file list and the file filter. */
+  // Tab cycles keyboard focus between the sidebar and the diff stream. The filter
+  // input is reached with `/`; Tab inside that input stays in the input so users
+  // can type the literal character without being kicked out.
   const toggleFocusArea = useCallback(() => {
-    setFocusArea((current) => (current === "files" ? "filter" : "files"));
+    setFocusArea((current) => (current === "diff" ? "files" : "diff"));
   }, []);
 
   /** Cycle through the available built-in themes. */
@@ -548,6 +549,7 @@ export function App({
     focusArea,
     focusFilter,
     moveToAnnotatedHunk,
+    moveToFile: review.moveToFile,
     moveToHunk: review.moveToHunk,
     moveMenuItem,
     openMenu,
@@ -673,6 +675,7 @@ export function App({
           <>
             <SidebarPane
               entries={review.sidebarEntries}
+              focused={focusArea === "files"}
               scrollRef={sidebarScrollRef}
               selectedFileId={selectedFile?.id}
               textWidth={sidebarTextWidth}
@@ -687,6 +690,7 @@ export function App({
             <PaneDivider
               dividerHitLeft={dividerHitLeft}
               dividerHitWidth={DIVIDER_HIT_WIDTH}
+              highlighted={focusArea === "files"}
               isResizing={isResizingSidebar}
               theme={activeTheme}
               onMouseDown={beginSidebarResize}
diff --git a/src/ui/AppHost.interactions.test.tsx b/src/ui/AppHost.interactions.test.tsx
index 82dc9e45..a189ebaf 100644
--- a/src/ui/AppHost.interactions.test.tsx
+++ b/src/ui/AppHost.interactions.test.tsx
@@ -1018,18 +1018,18 @@ describe("App interactions", () => {
         await setup.mockInput.pressKey("F10");
       });
       let frame = await waitForFrame(setup, (currentFrame) =>
-        currentFrame.includes("Toggle files/filter focus"),
+        currentFrame.includes("Toggle files/diff focus"),
       );
-      if (!frame.includes("Toggle files/filter focus")) {
+      if (!frame.includes("Toggle files/diff focus")) {
         await act(async () => {
           await setup.mockInput.pressKey("F10");
         });
         frame = await waitForFrame(setup, (currentFrame) =>
-          currentFrame.includes("Toggle files/filter focus"),
+          currentFrame.includes("Toggle files/diff focus"),
         );
       }
 
-      expect(frame).toContain("Toggle files/filter focus");
+      expect(frame).toContain("Toggle files/diff focus");
       expect(frame).toContain("Reload");
       expect(frame).toContain("Quit");
 
@@ -1690,6 +1690,57 @@ describe("App interactions", () => {
     }
   });
 
+  test("Tab cycles focus between sidebar and diff, and ↓ steps the file selection while sidebar holds focus", async () => {
+    const { getLatestSnapshot, hostClient } = createMockHostClient();
+    const setup = await testRender(
+      <AppHost bootstrap={createBootstrap()} hostClient={hostClient} />,
+      { width: 240, height: 24 },
+    );
+
+    try {
+      await flush(setup);
+
+      // Default focus is "diff": pressing ↓ scrolls the diff and leaves the
+      // file selection on alpha.ts.
+      expect(getLatestSnapshot()).toMatchObject({ selectedFilePath: "alpha.ts" });
+      await act(async () => {
+        await setup.mockInput.pressArrow("down");
+      });
+      await flush(setup);
+      expect(getLatestSnapshot()).toMatchObject({ selectedFilePath: "alpha.ts" });
+
+      // Tab once → sidebar holds focus, ↓ steps to the next visible file.
+      await act(async () => {
+        await setup.mockInput.pressTab();
+      });
+      await flush(setup);
+      await act(async () => {
+        await setup.mockInput.pressArrow("down");
+      });
+      const snapshotOnFiles = await waitForSnapshot(
+        setup,
+        getLatestSnapshot,
+        (snapshot) => snapshot.selectedFilePath === "beta.ts",
+      );
+      expect(snapshotOnFiles).toMatchObject({ selectedFilePath: "beta.ts" });
+
+      // Tab again → focus returns to the diff; ↓ no longer moves files.
+      await act(async () => {
+        await setup.mockInput.pressTab();
+      });
+      await flush(setup);
+      await act(async () => {
+        await setup.mockInput.pressArrow("down");
+      });
+      await flush(setup);
+      expect(getLatestSnapshot()).toMatchObject({ selectedFilePath: "beta.ts" });
+    } finally {
+      await act(async () => {
+        setup.renderer.destroy();
+      });
+    }
+  });
+
   test("filter focus accepts typed input and narrows the visible file set", async () => {
     const setup = await testRender(<AppHost bootstrap={createBootstrap()} />, {
       width: 240,
@@ -1700,7 +1751,7 @@ describe("App interactions", () => {
       await flush(setup);
 
       await act(async () => {
-        await setup.mockInput.pressTab();
+        await setup.mockInput.typeText("/");
       });
       await flush(setup);
       await act(async () => {
@@ -1729,7 +1780,7 @@ describe("App interactions", () => {
       await flush(setup);
 
       await act(async () => {
-        await setup.mockInput.pressTab();
+        await setup.mockInput.typeText("/");
       });
       await flush(setup);
       await act(async () => {
@@ -1744,7 +1795,7 @@ describe("App interactions", () => {
       expect(frame).not.toContain("Annotation for alpha.ts");
 
       await act(async () => {
-        await setup.mockInput.pressTab();
+        await setup.mockInput.pressEnter();
       });
       await flush(setup);
 
@@ -1772,7 +1823,7 @@ describe("App interactions", () => {
       await flush(setup);
 
       await act(async () => {
-        await setup.mockInput.pressTab();
+        await setup.mockInput.typeText("/");
       });
       await flush(setup);
       await act(async () => {
@@ -1863,7 +1914,7 @@ describe("App interactions", () => {
       await flush(setup);
 
       let frame = setup.captureCharFrame();
-      expect(frame).toContain("Toggle files/filter focus");
+      expect(frame).toContain("Toggle files/diff focus");
       expect(frame).not.toContain("Controls help");
 
       await act(async () => {
@@ -1873,7 +1924,7 @@ describe("App interactions", () => {
 
       frame = setup.captureCharFrame();
       expect(frame).toContain("Controls help");
-      expect(frame).not.toContain("Toggle files/filter focus");
+      expect(frame).not.toContain("Toggle files/diff focus");
 
       await act(async () => {
         await setup.mockInput.pressArrow("right");
@@ -1881,7 +1932,7 @@ describe("App interactions", () => {
       await flush(setup);
 
       frame = setup.captureCharFrame();
-      expect(frame).toContain("Toggle files/filter focus");
+      expect(frame).toContain("Toggle files/diff focus");
       expect(frame).not.toContain("Controls help");
     } finally {
       await act(async () => {
diff --git a/src/ui/AppHost.responsive.test.tsx b/src/ui/AppHost.responsive.test.tsx
index b26f10bf..ddcae131 100644
--- a/src/ui/AppHost.responsive.test.tsx
+++ b/src/ui/AppHost.responsive.test.tsx
@@ -216,7 +216,7 @@ describe("responsive app", () => {
     try {
       await act(async () => {
         await setup.renderOnce();
-        await setup.mockInput.pressTab();
+        await setup.mockInput.typeText("/");
         await setup.renderOnce();
       });
 
diff --git a/src/ui/components/chrome/HelpDialog.tsx b/src/ui/components/chrome/HelpDialog.tsx
index 1ed1f595..bc52b464 100644
--- a/src/ui/components/chrome/HelpDialog.tsx
+++ b/src/ui/components/chrome/HelpDialog.tsx
@@ -51,7 +51,7 @@ export function HelpDialog({
       title: "Review",
       items: [
         ["/", "focus file filter"],
-        ["Tab", "toggle files/filter focus"],
+        ["Tab", "toggle files/diff focus"],
         ["F10", "open menus"],
         [canRefresh ? "r / q" : "q", canRefresh ? "reload / quit" : "quit"],
       ],
diff --git a/src/ui/components/panes/PaneDivider.tsx b/src/ui/components/panes/PaneDivider.tsx
index c6a879ad..2836ba1c 100644
--- a/src/ui/components/panes/PaneDivider.tsx
+++ b/src/ui/components/panes/PaneDivider.tsx
@@ -5,6 +5,7 @@ import type { AppTheme } from "../../themes";
 export function PaneDivider({
   dividerHitLeft,
   dividerHitWidth,
+  highlighted = false,
   isResizing,
   theme,
   onMouseDown,
@@ -14,6 +15,7 @@ export function PaneDivider({
 }: {
   dividerHitLeft: number;
   dividerHitWidth: number;
+  highlighted?: boolean;
   isResizing: boolean;
   theme: AppTheme;
   onMouseDown: (event: TuiMouseEvent) => void;
@@ -21,13 +23,14 @@ export function PaneDivider({
   onMouseDragEnd: (event: TuiMouseEvent) => void;
   onMouseUp: (event: TuiMouseEvent) => void;
 }) {
+  const accent = isResizing || highlighted;
   return (
     <>
       <box
         style={{
           width: 1,
           border: ["top", "left"],
-          borderColor: isResizing ? theme.accent : theme.border,
+          borderColor: accent ? theme.accent : theme.border,
           backgroundColor: isResizing ? theme.accentMuted : theme.panel,
         }}
         customBorderChars={{
diff --git a/src/ui/components/panes/SidebarPane.tsx b/src/ui/components/panes/SidebarPane.tsx
index 0391c25a..b91b35a7 100644
--- a/src/ui/components/panes/SidebarPane.tsx
+++ b/src/ui/components/panes/SidebarPane.tsx
@@ -7,6 +7,7 @@ import { FileGroupHeader, FileListItem } from "./FileListItem";
 /** Render the file navigation sidebar. */
 export function SidebarPane({
   entries,
+  focused,
   scrollRef,
   selectedFileId,
   textWidth,
@@ -15,6 +16,7 @@ export function SidebarPane({
   onSelectFile,
 }: {
   entries: SidebarEntry[];
+  focused: boolean;
   scrollRef: RefObject<ScrollBoxRenderable | null>;
   selectedFileId?: string;
   textWidth: number;
@@ -30,7 +32,7 @@ export function SidebarPane({
       style={{
         width,
         border: ["top"],
-        borderColor: theme.border,
+        borderColor: focused ? theme.accent : theme.border,
         backgroundColor: theme.panel,
         paddingY: 1,
         paddingX: 0,
diff --git a/src/ui/components/ui-components.test.tsx b/src/ui/components/ui-components.test.tsx
index 668a3f15..0a26b2b1 100644
--- a/src/ui/components/ui-components.test.tsx
+++ b/src/ui/components/ui-components.test.tsx
@@ -335,6 +335,37 @@ async function captureFrame(node: ReactNode, width = 120, height = 24) {
   }
 }
 
+async function captureSpansFrame(node: ReactNode, width = 120, height = 24) {
+  const setup = await testRender(node, { width, height });
+
+  try {
+    await act(async () => {
+      await setup.renderOnce();
+    });
+
+    return setup.captureSpans();
+  } finally {
+    await act(async () => {
+      setup.renderer.destroy();
+    });
+  }
+}
+
+/** Pull the foreground colour of the first horizontal-border character on the
+ * top row of the rendered output. */
+function topBorderColor(frame: {
+  lines: Array<{ spans: Array<{ text: string; fg?: { buffer?: ArrayLike<number> } }> }>;
+}) {
+  const topLine = frame.lines[0];
+  if (!topLine) return null;
+  for (const span of topLine.spans) {
+    if (/[─━]/.test(span.text)) {
+      return capturedColorToHex(span.fg);
+    }
+  }
+  return null;
+}
+
 function frameHasHighlightedMarker(
   frame: { lines: Array<{ spans: Array<{ text: string; fg?: unknown; bg?: unknown }> }> },
   marker: string,
@@ -439,6 +470,7 @@ describe("UI components", () => {
     const frame = await captureFrame(
       <SidebarPane
         entries={buildSidebarEntries(files)}
+        focused={false}
         scrollRef={createRef()}
         selectedFileId="app"
         textWidth={28}
@@ -463,6 +495,53 @@ describe("UI components", () => {
     expect(frame).not.toContain("M +2 -1 AI");
   });
 
+  test("SidebarPane top-border colour shifts to theme.accent when focused", async () => {
+    const theme = resolveTheme("midnight", null);
+    const files = [
+      createTestDiffFile(
+        "app",
+        "src/ui/App.tsx",
+        "export const app = 1;\n",
+        "export const app = 2;\n",
+      ),
+    ];
+
+    const blurred = await captureSpansFrame(
+      <SidebarPane
+        entries={buildSidebarEntries(files)}
+        focused={false}
+        scrollRef={createRef()}
+        selectedFileId="app"
+        textWidth={28}
+        theme={theme}
+        width={32}
+        onSelectFile={() => {}}
+      />,
+      36,
+      6,
+    );
+    const focused = await captureSpansFrame(
+      <SidebarPane
+        entries={buildSidebarEntries(files)}
+        focused={true}
+        scrollRef={createRef()}
+        selectedFileId="app"
+        textWidth={28}
+        theme={theme}
+        width={32}
+        onSelectFile={() => {}}
+      />,
+      36,
+      6,
+    );
+
+    const blurredColor = topBorderColor(blurred);
+    const focusedColor = topBorderColor(focused);
+    expect(blurredColor).not.toBeNull();
+    expect(focusedColor).not.toBeNull();
+    expect(focusedColor).not.toBe(blurredColor);
+  });
+
   test("DiffPane renders all diff sections in file order", async () => {
     const bootstrap = createBootstrap();
     const theme = resolveTheme("midnight", null);
@@ -1618,7 +1697,7 @@ describe("UI components", () => {
       "l / w / m       lines / wrap / metadata",
       "Review",
       "/               focus file filter",
-      "Tab             toggle files/filter focus",
+      "Tab             toggle files/diff focus",
       "F10             open menus",
       "r / q           reload / quit",
     ] as const;
diff --git a/src/ui/hooks/useAppKeyboardShortcuts.ts b/src/ui/hooks/useAppKeyboardShortcuts.ts
index f2349d9e..45953d51 100644
--- a/src/ui/hooks/useAppKeyboardShortcuts.ts
+++ b/src/ui/hooks/useAppKeyboardShortcuts.ts
@@ -3,6 +3,7 @@ import { useKeyboard } from "@opentui/react";
 import { useRef } from "react";
 import type { LayoutMode } from "../../core/types";
 import type { MenuId } from "../components/chrome/menu";
+import type { FocusArea } from "../lib/focus";
 import {
   isEscapeKey,
   isHalfPageDownKey,
@@ -14,7 +15,6 @@ import {
   isStepUpKey,
 } from "../lib/keyboard";
 
-type FocusArea = "files" | "filter";
 type ScrollUnit = "step" | "viewport" | "content" | "half";
 
 const FAST_CODE_HORIZONTAL_SCROLL_COLUMNS = 8;
@@ -29,6 +29,7 @@ export interface UseAppKeyboardShortcutsOptions {
   focusArea: FocusArea;
   focusFilter: () => void;
   moveToAnnotatedHunk: (delta: number) => void;
+  moveToFile: (delta: number) => void;
   moveToHunk: (delta: number) => void;
   moveMenuItem: (delta: number) => void;
   openMenu: (menuId: MenuId) => void;
@@ -60,6 +61,7 @@ export function useAppKeyboardShortcuts({
   focusArea,
   focusFilter,
   moveToAnnotatedHunk,
+  moveToFile,
   moveToHunk,
   moveMenuItem,
   openMenu,
@@ -230,12 +232,10 @@ export function useAppKeyboardShortcuts({
       return false;
     }
 
-    if (key.name === "tab") {
-      toggleFocusArea();
-      return true;
-    }
-
-    // Let the focused input own filter editing and escape handling.
+    // The focused input owns every keystroke (including Tab) so users can type
+    // literal characters. Esc handling lives in StatusBar so it can
+    // preventDefault before the input swallows it as text.
+    void key;
     return true;
   };
 
@@ -296,13 +296,24 @@ export function useAppKeyboardShortcuts({
       return;
     }
 
+    // While the sidebar owns focus, treat step-up/down as file navigation rather than
+    // diff scrolling — the diff still follows because selectFile aligns the new file
+    // header to the top.
     if (isStepUpKey(key)) {
-      scrollDiff(-1, "step");
+      if (focusAreaRef.current === "files") {
+        moveToFile(-1);
+      } else {
+        scrollDiff(-1, "step");
+      }
       return;
     }
 
     if (isStepDownKey(key)) {
-      scrollDiff(1, "step");
+      if (focusAreaRef.current === "files") {
+        moveToFile(1);
+      } else {
+        scrollDiff(1, "step");
+      }
       return;
     }
 
diff --git a/src/ui/hooks/useReviewController.test.tsx b/src/ui/hooks/useReviewController.test.tsx
index d0885f59..18e18ed7 100644
--- a/src/ui/hooks/useReviewController.test.tsx
+++ b/src/ui/hooks/useReviewController.test.tsx
@@ -363,4 +363,55 @@ describe("useReviewController", () => {
       });
     }
   });
+
+  test("moveToFile steps the selection forward and back across visible files", async () => {
+    const controllerRef: { current: ReviewController | null } = { current: null };
+    const setup = await testRender(
+      <ReviewControllerHarness
+        initialFiles={[
+          createDiffFile("alpha", "alpha.ts", "export const a = 1;\n", "export const a = 2;\n"),
+          createDiffFile("beta", "beta.ts", "export const b = 1;\n", "export const b = 2;\n"),
+          createDiffFile("gamma", "gamma.ts", "export const g = 1;\n", "export const g = 2;\n"),
+        ]}
+        onController={(nextController) => {
+          controllerRef.current = nextController;
+        }}
+      />,
+      { width: 80, height: 4 },
+    );
+
+    try {
+      await flush(setup);
+      expect(expectValue(controllerRef.current).selectedFileId).toBe("alpha");
+
+      await act(async () => {
+        expectValue(controllerRef.current).moveToFile(1);
+      });
+      await flush(setup);
+      expect(expectValue(controllerRef.current).selectedFileId).toBe("beta");
+
+      await act(async () => {
+        expectValue(controllerRef.current).moveToFile(1);
+      });
+      await flush(setup);
+      expect(expectValue(controllerRef.current).selectedFileId).toBe("gamma");
+
+      // Past the last file the selection clamps; we don't wrap around.
+      await act(async () => {
+        expectValue(controllerRef.current).moveToFile(1);
+      });
+      await flush(setup);
+      expect(expectValue(controllerRef.current).selectedFileId).toBe("gamma");
+
+      await act(async () => {
+        expectValue(controllerRef.current).moveToFile(-2);
+      });
+      await flush(setup);
+      expect(expectValue(controllerRef.current).selectedFileId).toBe("alpha");
+    } finally {
+      await act(async () => {
+        setup.renderer.destroy();
+      });
+    }
+  });
 });
diff --git a/src/ui/hooks/useReviewController.ts b/src/ui/hooks/useReviewController.ts
index cd2508dc..5a540629 100644
--- a/src/ui/hooks/useReviewController.ts
+++ b/src/ui/hooks/useReviewController.ts
@@ -32,6 +32,7 @@ import type {
   RemovedCommentResult,
   SessionLiveCommentSummary,
 } from "../../hunk-session/types";
+import { stepFileInList } from "../lib/files";
 import { findNextHunkCursor } from "../lib/hunks";
 import {
   buildReviewState,
@@ -60,6 +61,7 @@ export interface ReviewController {
   liveCommentsByFileId: Record<string, LiveComment[]>;
   moveToAnnotatedFile: (delta: number) => void;
   moveToAnnotatedHunk: (delta: number) => void;
+  moveToFile: (delta: number) => void;
   moveToHunk: (delta: number) => void;
   scrollToNote: boolean;
   selectedFile: DiffFile | undefined;
@@ -247,6 +249,18 @@ export function useReviewController({ files }: { files: DiffFile[] }): ReviewCon
     [selectFile, selectedFile?.id, visibleFiles],
   );
 
+  /** Step the file selection by `delta` across the currently visible review stream. */
+  const moveToFile = useCallback(
+    (delta: number) => {
+      const nextFile = stepFileInList(visibleFiles, selectedFile?.id, delta, "clamp");
+      if (!nextFile) {
+        return;
+      }
+      selectFile(nextFile.id, 0, { alignFileHeaderTop: true });
+    },
+    [selectFile, selectedFile?.id, visibleFiles],
+  );
+
   /** Clear the active file filter without touching the current selection. */
   const clearFilter = useCallback(() => {
     setFilter("");
@@ -504,6 +518,7 @@ export function useReviewController({ files }: { files: DiffFile[] }): ReviewCon
     clearLiveComments,
     moveToAnnotatedFile,
     moveToAnnotatedHunk,
+    moveToFile,
     moveToHunk,
     navigateToLocation,
     removeLiveComment,
diff --git a/src/ui/lib/appMenus.ts b/src/ui/lib/appMenus.ts
index 058bb33c..89f735cf 100644
--- a/src/ui/lib/appMenus.ts
+++ b/src/ui/lib/appMenus.ts
@@ -66,7 +66,7 @@ export function buildAppMenus({
   const fileMenuEntries: MenuEntry[] = [
     {
       kind: "item",
-      label: "Toggle files/filter focus",
+      label: "Toggle files/diff focus",
       hint: "Tab",
       action: toggleFocusArea,
     },
diff --git a/src/ui/lib/files.test.ts b/src/ui/lib/files.test.ts
index 4e2b653f..94045320 100644
--- a/src/ui/lib/files.test.ts
+++ b/src/ui/lib/files.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 import { createTestDiffFile, lines } from "../../../test/helpers/diff-helpers";
-import { buildSidebarEntries, fileLabelParts } from "./files";
+import { buildSidebarEntries, fileLabelParts, stepFileInList } from "./files";
 
 describe("files helpers", () => {
   test("buildSidebarEntries hides zero-value sidebar stats", () => {
@@ -117,6 +117,59 @@ describe("files helpers", () => {
     });
   });
 
+  describe("stepFileInList", () => {
+    const stub = (id: string) =>
+      ({ id }) as unknown as Parameters<typeof stepFileInList>[0][number];
+
+    test("returns null for an empty list", () => {
+      expect(stepFileInList([], "anything", 1, "clamp")).toBeNull();
+      expect(stepFileInList([], "anything", -1, "wrap")).toBeNull();
+    });
+
+    test("returns null when delta is zero", () => {
+      const files = [stub("a"), stub("b")];
+      expect(stepFileInList(files, "a", 0, "clamp")).toBeNull();
+    });
+
+    test("clamp mode steps forward and backward", () => {
+      const files = [stub("a"), stub("b"), stub("c")];
+      expect(stepFileInList(files, "a", 1, "clamp")?.id).toBe("b");
+      expect(stepFileInList(files, "b", 1, "clamp")?.id).toBe("c");
+      expect(stepFileInList(files, "b", -1, "clamp")?.id).toBe("a");
+    });
+
+    test("clamp mode returns null at the boundary instead of moving", () => {
+      const files = [stub("a"), stub("b"), stub("c")];
+      expect(stepFileInList(files, "c", 1, "clamp")).toBeNull();
+      expect(stepFileInList(files, "a", -1, "clamp")).toBeNull();
+    });
+
+    test("clamp mode lands on the first file when nothing is selected", () => {
+      // Edge case the inline implementation got wrong: a forward step with no
+      // current selection used to skip index 0 because baseIndex defaulted to 0.
+      const files = [stub("a"), stub("b"), stub("c")];
+      expect(stepFileInList(files, undefined, 1, "clamp")?.id).toBe("a");
+      expect(stepFileInList(files, "missing", 1, "clamp")?.id).toBe("a");
+    });
+
+    test("clamp mode lands on the last file when stepping back with nothing selected", () => {
+      const files = [stub("a"), stub("b"), stub("c")];
+      expect(stepFileInList(files, undefined, -1, "clamp")?.id).toBe("c");
+    });
+
+    test("wrap mode cycles past either end", () => {
+      const files = [stub("a"), stub("b"), stub("c")];
+      expect(stepFileInList(files, "c", 1, "wrap")?.id).toBe("a");
+      expect(stepFileInList(files, "a", -1, "wrap")?.id).toBe("c");
+    });
+
+    test("wrap mode returns null when only one file exists", () => {
+      const files = [stub("solo")];
+      expect(stepFileInList(files, "solo", 1, "wrap")).toBeNull();
+      expect(stepFileInList(files, "solo", -1, "wrap")).toBeNull();
+    });
+  });
+
   test("fileLabelParts strips parser-added line endings from rename labels", () => {
     const renamedAcrossDirectories = {
       ...createTestDiffFile({
diff --git a/src/ui/lib/files.ts b/src/ui/lib/files.ts
index 562860fc..d9ad5512 100644
--- a/src/ui/lib/files.ts
+++ b/src/ui/lib/files.ts
@@ -154,6 +154,46 @@ export function buildSidebarEntries(files: DiffFile[]): SidebarEntry[] {
   return entries;
 }
 
+/**
+ * Step through `files` by `delta` based on `currentFileId`.
+ *
+ * Returns the file the new selection should land on, or `null` if no movement
+ * is possible (empty list, "clamp" already at the boundary, or a single-item
+ * list cycled in "wrap" mode).
+ *
+ * When `currentFileId` is not in the list, the cursor is treated as sitting
+ * just outside the list in the direction opposite `delta`, so the first
+ * forward step lands on index 0 (not 1) and the first backward step lands on
+ * the last entry — what the user expects when nothing is currently selected.
+ */
+export function stepFileInList(
+  files: DiffFile[],
+  currentFileId: string | undefined,
+  delta: number,
+  mode: "clamp" | "wrap",
+): DiffFile | null {
+  if (files.length === 0 || delta === 0) {
+    return null;
+  }
+
+  const currentIndex = files.findIndex((file) => file.id === currentFileId);
+  const baseIndex = currentIndex < 0 ? (delta > 0 ? -1 : files.length) : currentIndex;
+
+  let nextIndex: number;
+  if (mode === "wrap") {
+    const length = files.length;
+    nextIndex = (((baseIndex + delta) % length) + length) % length;
+  } else {
+    nextIndex = Math.min(Math.max(0, baseIndex + delta), files.length - 1);
+  }
+
+  if (nextIndex === currentIndex) {
+    return null;
+  }
+
+  return files[nextIndex] ?? null;
+}
+
 /** Build the canonical file label used across headers and note cards. */
 export function fileLabel(file: DiffFile | undefined) {
   const { filename, stateLabel } = fileLabelParts(file);
diff --git a/src/ui/lib/focus.ts b/src/ui/lib/focus.ts
new file mode 100644
index 00000000..80d6580f
--- /dev/null
+++ b/src/ui/lib/focus.ts
@@ -0,0 +1,12 @@
+/**
+ * Which app surface currently owns keyboard input.
+ *
+ * - `"diff"`: the main review stream (default on launch). Step keys scroll
+ *   the diff one row at a time.
+ * - `"files"`: the sidebar file list. Step keys move the file selection
+ *   instead of scrolling.
+ * - `"filter"`: the file filter input is active. The input swallows every
+ *   keystroke (including Tab) so users can type literal characters; Esc
+ *   exits the input.
+ */
+export type FocusArea = "files" | "diff" | "filter";
diff --git a/src/ui/lib/reviewState.ts b/src/ui/lib/reviewState.ts
index c5a647ba..aad0ecac 100644
--- a/src/ui/lib/reviewState.ts
+++ b/src/ui/lib/reviewState.ts
@@ -17,6 +17,7 @@ import {
   buildSidebarEntries,
   filterReviewFiles,
   mergeFileAnnotationsByFileId,
+  stepFileInList,
   type SidebarEntry,
 } from "./files";
 import {
@@ -106,14 +107,7 @@ export function findNextAnnotatedFile(
   delta: number,
 ) {
   const annotatedFiles = visibleFiles.filter((file) => file.agent);
-  if (annotatedFiles.length === 0) {
-    return null;
-  }
-
-  const currentIndex = annotatedFiles.findIndex((file) => file.id === currentFileId);
-  const normalizedIndex = currentIndex >= 0 ? currentIndex : 0;
-  const nextIndex = (normalizedIndex + delta + annotatedFiles.length) % annotatedFiles.length;
-  return annotatedFiles[nextIndex] ?? null;
+  return stepFileInList(annotatedFiles, currentFileId, delta, "wrap");
 }
 
 /** Resolve one session-daemon navigation request against the review stream's current state. */
diff --git a/src/ui/lib/ui-lib.test.ts b/src/ui/lib/ui-lib.test.ts
index 98c77b8f..1c9f176e 100644
--- a/src/ui/lib/ui-lib.test.ts
+++ b/src/ui/lib/ui-lib.test.ts
@@ -151,10 +151,10 @@ describe("ui helpers", () => {
       menus.file
         .filter((entry): entry is Extract<MenuEntry, { kind: "item" }> => entry.kind === "item")
         .map((entry) => entry.label),
-    ).toEqual(["Toggle files/filter focus", "Focus filter", "Reload", "Quit"]);
+    ).toEqual(["Toggle files/diff focus", "Focus filter", "Reload", "Quit"]);
     expect(menus.file[0]).toMatchObject({
       kind: "item",
-      label: "Toggle files/filter focus",
+      label: "Toggle files/diff focus",
       hint: "Tab",
     });
     expect(
diff --git a/test/pty/ui-integration.test.ts b/test/pty/ui-integration.test.ts
index cd769913..1316a14b 100644
--- a/test/pty/ui-integration.test.ts
+++ b/test/pty/ui-integration.test.ts
@@ -506,7 +506,7 @@ describe("live UI integration", () => {
       expect(initial).toContain("add = true");
       expect(initial).toContain("betaValue");
 
-      await session.press("tab");
+      await session.type("/");
       await session.type("beta");
       const filtered = await harness.waitForSnapshot(
         session,
@@ -917,7 +917,7 @@ describe("live UI integration", () => {
       await session.press("f10");
       const fileMenu = await harness.waitForSnapshot(
         session,
-        (text) => text.includes("Toggle files/filter focus") && text.includes("Quit"),
+        (text) => text.includes("Toggle files/diff focus") && text.includes("Quit"),
         5_000,
       );