diff --git a/app/globals.css b/app/globals.css
index 4010ec13d4..2fc77b0d34 100644
--- a/app/globals.css
+++ b/app/globals.css
@@ -1676,6 +1676,48 @@ span.token.table {
   }
 }
 
+/* Full-width mode keeps the mounted viewer and saved split width intact. */
+@media (min-width: 641px) {
+  .right-panel-container.right-panel-open.right-panel-full-width {
+    position: fixed;
+    inset: 0 env(safe-area-inset-right) 0 env(safe-area-inset-left);
+    z-index: 250;
+    width: auto;
+    min-width: 0;
+    height: var(--app-viewport-height, 100dvh);
+    border-left: none !important;
+    box-shadow: none;
+    transform: none;
+    transition: none;
+  }
+  .right-panel-container.right-panel-full-width > * {
+    width: 100%;
+    min-width: 0;
+  }
+}
+.file-panel-expand-button {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  flex-shrink: 0;
+  width: 36px;
+  height: 36px;
+  padding: 0;
+  background: var(--bg-selected);
+  border: none;
+  border-left: 1px solid var(--border);
+  color: var(--text);
+  cursor: pointer;
+}
+.file-panel-expand-button:hover,
+.file-panel-expand-button[aria-pressed="true"] {
+  color: var(--accent);
+}
+.file-panel-expand-button:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: -3px;
+}
+
 /* Mobile sidebar: slide in/out as overlay */
 @media (max-width: 640px) {
   .chat-stats-center {
@@ -1722,6 +1764,7 @@ span.token.table {
     width: 100%;
     min-width: 0;
   }
+  .file-panel-expand-button,
   .panel-resize-handle,
   .right-panel-overlay-backdrop {
     display: none;
diff --git a/components/AppShell.mobile-toolbar.test.mjs b/components/AppShell.mobile-toolbar.test.mjs
index e95c32add2..536d70139f 100644
--- a/components/AppShell.mobile-toolbar.test.mjs
+++ b/components/AppShell.mobile-toolbar.test.mjs
@@ -99,3 +99,12 @@ test("places trust warnings below the mobile toolbar and the file toggle in tool
   assert.doesNotMatch(source, /File panel toggle — always visible at top-right/);
   assert.doesNotMatch(source, /position: "fixed", top: "env\(safe-area-inset-top\)"/);
 });
+
+test("closes top-bar dropdowns when the file panel expands to full width", () => {
+  assert.match(
+    source,
+    /const handleRightPanelExpandToggle = useCallback\(\(\) => \{\s*setActiveTopPanel\(null\);\s*setRightPanelExpanded\(\(expanded\) => !expanded\);/,
+  );
+  assert.match(source, /onClick=\{handleRightPanelExpandToggle\}/);
+  assert.match(source, /if \(rightPanelFullWidth\) setActiveTopPanel\(null\);/);
+});
diff --git a/components/AppShell.tsx b/components/AppShell.tsx
index 86a1f4d426..514dbadb93 100644
--- a/components/AppShell.tsx
+++ b/components/AppShell.tsx
@@ -172,6 +172,11 @@ export function AppShell() {
   const [projectTrustError, setProjectTrustError] = useState<string | null>(null);
   const [sidebarOpen, setSidebarOpen] = useState(true);
   const [rightPanelOpen, setRightPanelOpen] = useState(false);
+  const [rightPanelExpanded, setRightPanelExpanded] = useState(false);
+  const rightPanelFullWidth = rightPanelOpen && rightPanelExpanded && !isMobile;
+  useEffect(() => {
+    if (!rightPanelOpen || isMobile) setRightPanelExpanded(false);
+  }, [rightPanelOpen, isMobile]);
   const [mobileToolbarMoreOpen, setMobileToolbarMoreOpen] = useState(false);
   const [mobileSidebarReady, setMobileSidebarReady] = useState(false);
   const sidebarWidthRef = useRef(SIDEBAR_DEFAULT_WIDTH);
@@ -334,6 +339,10 @@ export function AppShell() {
     }
   }, [hasSubagentSessions]);
 
+  useEffect(() => {
+    if (rightPanelFullWidth) setActiveTopPanel(null);
+  }, [rightPanelFullWidth]);
+
   const toggleTopPanel = useCallback((
     panel: "agents" | "branches" | "system" | "tools" | "session" | "language" | "theme",
     keepMobileToolbarOpen = false,
@@ -393,6 +402,11 @@ export function AppShell() {
     setRightPanelOpen((open) => !open);
   }, [isMobile]);
 
+  const handleRightPanelExpandToggle = useCallback(() => {
+    setActiveTopPanel(null);
+    setRightPanelExpanded((expanded) => !expanded);
+  }, []);
+
   useEffect(() => {
     if (!mobileToolbarMoreOpen) return;
 
@@ -1982,6 +1996,7 @@ export function AppShell() {
       <div
         ref={sidebarResizer.panelRef}
         id="session-sidebar"
+        inert={rightPanelFullWidth}
         className={`sidebar-container${sidebarOpen ? " sidebar-open" : " sidebar-closed"}${mobileSidebarReady ? "" : " sidebar-mobile-pending"}${sidebarResizer.isResizing ? " sidebar-resizing" : ""}`}
         style={{
           "--sidebar-width": `${sidebarResizer.width}px`,
@@ -2000,6 +2015,7 @@ export function AppShell() {
       {sidebarOpen && (
         <div
           {...sidebarResizer.separatorProps}
+          inert={rightPanelFullWidth}
           aria-controls="session-sidebar"
           className={`panel-resize-handle sidebar-resize-handle${sidebarResizer.isResizing ? " is-resizing" : ""}`}
           data-resize-handle="sidebar"
@@ -2008,7 +2024,7 @@ export function AppShell() {
       )}
 
       {/* Center: chat */}
-      <div style={{ flex: 1, display: "flex", flexDirection: "column", overflow: "hidden", minWidth: 0 }}>
+      <div inert={rightPanelFullWidth} style={{ flex: 1, display: "flex", flexDirection: "column", overflow: "hidden", minWidth: 0 }}>
         {/* Top bar with sidebar toggle */}
         <div ref={topBarRef} style={{ flexShrink: 0, background: "var(--bg-panel)" }}>
         <div style={{ display: "flex", alignItems: "center", position: "relative", borderBottom: "1px solid var(--border)", height: "calc(36px + env(safe-area-inset-top))", paddingTop: "env(safe-area-inset-top)" }}>
@@ -2558,6 +2574,7 @@ export function AppShell() {
       {rightPanelOpen && (
         <div
           {...rightPanelResizer.separatorProps}
+          inert={rightPanelFullWidth}
           aria-controls="file-panel"
           className={`panel-resize-handle right-panel-resize-handle${rightPanelResizer.isResizing ? " is-resizing" : ""}`}
           data-resize-handle="right-panel"
@@ -2569,7 +2586,7 @@ export function AppShell() {
       <div
         ref={rightPanelResizer.panelRef}
         id="file-panel"
-        className={`right-panel-container${rightPanelOpen ? " right-panel-open" : " right-panel-closed"}${rightPanelResizer.isResizing ? " right-panel-resizing" : ""}`}
+        className={`right-panel-container${rightPanelOpen ? " right-panel-open" : " right-panel-closed"}${rightPanelFullWidth ? " right-panel-full-width" : ""}${rightPanelResizer.isResizing ? " right-panel-resizing" : ""}`}
         style={{
           "--right-panel-width": `${rightPanelResizer.width}px`,
           display: "flex",
@@ -2596,6 +2613,21 @@ export function AppShell() {
               onCloseTab={handleCloseFileTab}
             />
           </div>
+          <button
+            type="button"
+            className="file-panel-expand-button"
+            onClick={handleRightPanelExpandToggle}
+            aria-controls="file-panel"
+            aria-pressed={rightPanelFullWidth}
+            title={translate(rightPanelFullWidth ? "files.restorePanelWidth" : "files.expandPanel")}
+            aria-label={translate(rightPanelFullWidth ? "files.restorePanelWidth" : "files.expandPanel")}
+          >
+            <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="1.7" strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
+              <path d={rightPanelFullWidth
+                ? "M9 3v6H3m12-6v6h6M9 21v-6H3m12 6v-6h6M3 3l6 6m12-6-6 6M3 21l6-6m12 6-6-6"
+                : "M8 3H3v5m13-5h5v5M3 16v5h5m13-5v5h-5M3 3l6 6m12-6-6 6M3 21l6-6m12 6-6-6"} />
+            </svg>
+          </button>
           <button
             type="button"
             onClick={() => setRightPanelOpen(false)}
diff --git a/e2e/file-panel.mjs b/e2e/file-panel.mjs
new file mode 100644
index 0000000000..d5ba6052b1
--- /dev/null
+++ b/e2e/file-panel.mjs
@@ -0,0 +1,66 @@
+import assert from "node:assert/strict";
+
+export const filePanelFixture = `<!doctype html><html><body style="margin:20px;min-height:2400px">
+<label>Notes <input id="notes"></label>
+<label>Filter <select id="filter"><option>All</option><option>Pending</option></select></label>
+<p>HTML preview state must survive layout changes.</p>
+<script>window.previewInstance = Math.random();</script></body></html>`;
+
+export async function checkFilePanel(page, filePath) {
+  const showSidebar = page.getByRole("button", { name: "Show sidebar", exact: true });
+  if (await showSidebar.isVisible()) await showSidebar.click();
+  await page.locator(`[title="${filePath}"]`).click();
+  const panel = page.locator("#file-panel");
+  const iframe = panel.locator("iframe");
+  await iframe.waitFor();
+  const frame = await (await iframe.elementHandle()).contentFrame();
+  await frame.locator("#notes").fill("Keep this note");
+  await frame.locator("#filter").selectOption({ label: "Pending" });
+  await frame.evaluate(() => scrollTo(0, 120));
+  const instance = await frame.evaluate(() => window.previewInstance);
+  const width = () => panel.evaluate(async el => {
+    await new Promise(requestAnimationFrame);
+    await Promise.all(el.getAnimations().map(animation => animation.finished.catch(() => {})));
+    return el.getBoundingClientRect().width;
+  });
+  const storedWidths = () => page.evaluate(() => [
+    localStorage.getItem("pi-sidebar-width"), localStorage.getItem("pi-right-panel-width"),
+  ]);
+  const toggle = panel.getByRole("button", { name: "Expand file panel", exact: true });
+  if (page.viewportSize().width <= 640) {
+    assert.equal(await toggle.isVisible(), false, "Mobile already uses full width");
+  } else {
+    // Include a manually resized split in the round trip.
+    const separator = page.locator('[data-resize-handle="right-panel"]');
+    if (await separator.isVisible()) await separator.press("ArrowLeft");
+    const originalWidth = await width();
+    const originalStored = await storedWidths();
+    const sessionInfo = page.getByRole("button", { name: "Session info", exact: true });
+    const sessionPopover = page.locator(".session-info-popover");
+    for (let i = 0; i < 2; i++) {
+      await sessionInfo.click();
+      await sessionPopover.waitFor();
+      await toggle.click();
+      assert.equal(await sessionPopover.count(), 0, "Full-width mode dismisses inert top-bar menus");
+      assert.equal(await panel.getByRole("button", { name: "Restore file panel width", exact: true }).getAttribute("aria-pressed"), "true");
+      assert.equal(Math.round(await width()), page.viewportSize().width);
+      assert.equal(await page.locator("#session-sidebar").evaluate(el => el.inert), true);
+      assert.equal(await frame.evaluate(() => window.previewInstance), instance);
+      assert.equal(await frame.evaluate(() => scrollY), 120);
+      assert.equal(await frame.locator("#notes").inputValue(), "Keep this note");
+      assert.equal(await frame.locator("#filter").inputValue(), "Pending");
+      await panel.getByRole("button", { name: "Restore file panel width", exact: true }).press("Enter");
+      assert.equal(await width(), originalWidth);
+      assert.deepEqual(await storedWidths(), originalStored);
+      assert.equal(await page.locator("#session-sidebar").evaluate(el => el.inert), false);
+    }
+    await toggle.click();
+    await panel.getByRole("button", { name: "Hide file panel", exact: true }).click();
+    await page.getByRole("button", { name: "Show file panel", exact: true }).click();
+    assert.equal(await toggle.getAttribute("aria-pressed"), "false", "Reopening returns to the split layout");
+    assert.equal(await width(), originalWidth);
+    assert.equal(await frame.evaluate(() => window.previewInstance), instance);
+  }
+  await panel.getByRole("button", { name: "Hide file panel", exact: true }).click();
+  console.log(`PASS: file panel width and preview state at ${page.viewportSize().width}px`);
+}
diff --git a/e2e/run.mjs b/e2e/run.mjs
index eba0386945..e8ace49583 100644
--- a/e2e/run.mjs
+++ b/e2e/run.mjs
@@ -9,6 +9,7 @@ import { dirname, join } from "node:path";
 import { setTimeout as delay } from "node:timers/promises";
 import { fileURLToPath } from "node:url";
 import { chromium } from "playwright";
+import { checkFilePanel, filePanelFixture } from "./file-panel.mjs";
 import { checkExtensionDialogs, extensionSource } from "./extension-dialog.mjs";
 import { checkChatAppearance } from "./chat-appearance.mjs";
 
@@ -22,6 +23,8 @@ const agentDir = mkdtempSync(join(tmpdir(), "pi-web-e2e-"));
 const project = join(agentDir, "project");
 const sessionDir = join(agentDir, "sessions", "e2e");
 mkdirSync(project);
+const previewFile = join(project, "preview.html");
+writeFileSync(previewFile, filePanelFixture);
 mkdirSync(sessionDir, { recursive: true });
 const timestamp = "2026-08-23T00:00:00.000Z";
 const LONG = "e2e-long-session";
@@ -351,6 +354,9 @@ try {
       await page.goto(`${base}/?session=${COMPACTED}`, { waitUntil: "domcontentloaded" });
       await heading.waitFor({ state: "visible" });
     }
+    await page.goto(`${base}/?session=${RICH}`, { waitUntil: "domcontentloaded" });
+    await page.locator(".markdown-code-block pre").waitFor();
+    await checkFilePanel(page, previewFile);
     await checkExtensionDialogs(page, artifacts, viewport.width);
     if (viewport.width > 600) {
       await page.goto(`${base}/?session=${RICH}`, { waitUntil: "domcontentloaded" });
diff --git a/lib/i18n/messages/en.ts b/lib/i18n/messages/en.ts
index 0e793a70db..f2a987e536 100644
--- a/lib/i18n/messages/en.ts
+++ b/lib/i18n/messages/en.ts
@@ -187,6 +187,8 @@ export const enLocale: LocalePlugin = {
     "workspace.stepProject": "Select a project directory from the sidebar",
     "workspace.stepModels": "Open Settings at the bottom, then choose Models",
     "files.hidePanel": "Hide file panel",
+    "files.expandPanel": "Expand file panel",
+    "files.restorePanelWidth": "Restore file panel width",
     "files.showPanel": "Show file panel",
     "files.noneOpen": "No file open",
     "layout.resizeSidebar": "Resize sidebar",
diff --git a/lib/i18n/messages/zh-CN.ts b/lib/i18n/messages/zh-CN.ts
index 3f4fb55f67..eb2ea62973 100644
--- a/lib/i18n/messages/zh-CN.ts
+++ b/lib/i18n/messages/zh-CN.ts
@@ -187,6 +187,8 @@ export const zhCNLocale: LocalePlugin = {
     "workspace.stepProject": "从侧边栏选择项目目录",
     "workspace.stepModels": "打开底部的“设置”，然后选择“模型”",
     "files.hidePanel": "隐藏文件面板",
+    "files.expandPanel": "全宽展示文件面板",
+    "files.restorePanelWidth": "还原文件面板宽度",
     "files.showPanel": "显示文件面板",
     "files.noneOpen": "没有打开的文件",
     "layout.resizeSidebar": "调整侧边栏宽度",
diff --git a/lib/i18n/messages/zh-TW.ts b/lib/i18n/messages/zh-TW.ts
index e2df0d3c03..f1a4b58b5b 100644
--- a/lib/i18n/messages/zh-TW.ts
+++ b/lib/i18n/messages/zh-TW.ts
@@ -187,6 +187,8 @@ export const zhTWLocale: LocalePlugin = {
     "workspace.stepProject": "從側邊欄選擇專案目錄",
     "workspace.stepModels": "開啟底部的「設定」，然後選擇「模型」",
     "files.hidePanel": "隱藏檔案面板",
+    "files.expandPanel": "全寬顯示檔案面板",
+    "files.restorePanelWidth": "還原檔案面板寬度",
     "files.showPanel": "顯示檔案面板",
     "files.noneOpen": "沒有開啟的檔案",
     "layout.resizeSidebar": "調整側邊欄寬度",