diff --git a/src/app/globals.css b/src/app/globals.css
index 409fa258a1..16f8e6bb0c 100644
--- a/src/app/globals.css
+++ b/src/app/globals.css
@@ -1528,6 +1528,15 @@ summary::-webkit-details-marker {
     pointer-events: none;
   }
 
+  /* Document viewer bottom composer mirrors the dock's hide-on-scroll. Unlike
+     the full-bleed dock it floats with a bottom gap (.floating-composer-edge,
+     bottom: max(0.75rem, safe-area)), so the exit transform must also clear
+     that gap. translateY-only: this form centres with mx-auto, not translateX. */
+  .document-viewer-composer[data-scroll-hidden="true"] {
+    transform: translateY(calc(100% + max(0.75rem, var(--safe-area-bottom))));
+    pointer-events: none;
+  }
+
   .answer-footer-search-dock .answer-footer-search-pill {
     border-color: var(--border-strong);
     background: var(--surface);
diff --git a/src/components/DocumentViewer.tsx b/src/components/DocumentViewer.tsx
index 630790c43f..e0d618a748 100644
--- a/src/components/DocumentViewer.tsx
+++ b/src/components/DocumentViewer.tsx
@@ -43,6 +43,7 @@ import {
   documentFileKind,
   documentTileTone,
 } from "@/components/clinical-dashboard/document-ui";
+import { useHideOnScroll } from "@/components/clinical-dashboard/use-hide-on-scroll";
 import { DocumentTagCloud } from "@/components/DocumentTagCloud";
 import type { PDFDocumentLoadingTask, PDFDocumentProxy, RenderTask } from "pdfjs-dist";
 import {
@@ -1916,6 +1917,12 @@ export function DocumentViewer({
   const [isOnline, setIsOnline] = useState(true);
   const [localProjectReady, setLocalProjectReady] = useState(true);
   const [mobileActionsOpen, setMobileActionsOpen] = useState(false);
+  // Phone-only hide-on-scroll for the bottom composer: never hide while the
+  // mobile actions sheet is open or while focus sits inside the composer
+  // (keyboard users must not tab into invisible controls).
+  const [composerChromeFocused, setComposerChromeFocused] = useState(false);
+  const scrollHidden = useHideOnScroll({});
+  const composerScrollHidden = scrollHidden && !mobileActionsOpen && !composerChromeFocused;
   const [useNativePdfViewer, setUseNativePdfViewer] = useState(() => getInitialPdfViewerMode().useNativePdfViewer);
   const [hasExplicitPdfViewerMode, setHasExplicitPdfViewerMode] = useState(
     () => getInitialPdfViewerMode().hasExplicitPdfViewerMode,
@@ -2964,7 +2971,12 @@ export function DocumentViewer({
             event.preventDefault();
             if (canSummarizeDocument) void summarize();
           }}
-          className="floating-composer-edge dashboard-composer-edge fixed z-40 mx-auto flex min-h-[56px] max-w-3xl items-center gap-2 rounded-full border border-[color:var(--border-lux)] bg-[color:var(--surface-lux)] px-2 shadow-[var(--shadow-lux)] ring-1 ring-white/35 backdrop-blur-xl"
+          data-scroll-hidden={composerScrollHidden ? "true" : undefined}
+          onFocusCapture={() => setComposerChromeFocused(true)}
+          onBlurCapture={(event) => {
+            if (!event.currentTarget.contains(event.relatedTarget as Node | null)) setComposerChromeFocused(false);
+          }}
+          className="document-viewer-composer floating-composer-edge dashboard-composer-edge fixed z-40 mx-auto flex min-h-[56px] max-w-3xl items-center gap-2 rounded-full border border-[color:var(--border-lux)] bg-[color:var(--surface-lux)] px-2 shadow-[var(--shadow-lux)] ring-1 ring-white/35 backdrop-blur-xl max-sm:transition-transform max-sm:duration-200 max-sm:ease-out motion-reduce:transition-none"
         >
           <button
             type="button"
diff --git a/tests/ui-smoke.spec.ts b/tests/ui-smoke.spec.ts
index 98b23c05e8..7055c81838 100644
--- a/tests/ui-smoke.spec.ts
+++ b/tests/ui-smoke.spec.ts
@@ -2001,6 +2001,34 @@ test.describe("Clinical KB UI smoke coverage", () => {
     await expectNoPageHorizontalOverflow(page);
   });
 
+  test("document viewer bottom composer hides while scrolling down on phones", async ({ page }) => {
+    await page.setViewportSize({ width: 390, height: 844 });
+    await mockDemoApi(page);
+    await gotoApp(
+      page,
+      "/documents/11111111-1111-4111-8111-111111111111?page=1&chunk=44444444-4444-4444-8444-444444444442",
+    );
+
+    await expect(page.getByRole("heading", { level: 1, name: "Synthetic lithium monitoring protocol" })).toBeVisible();
+    const composer = page.locator("form.document-viewer-composer");
+    await expect(composer).toBeVisible();
+    await expect(composer).not.toHaveAttribute("data-scroll-hidden", "true");
+
+    // Hide on deliberate scroll down past the activation offset.
+    await page.evaluate(() => window.scrollTo({ top: 120, behavior: "auto" }));
+    await expect(composer).toHaveAttribute("data-scroll-hidden", "true");
+
+    // Reappear on scroll up.
+    await page.evaluate(() => window.scrollTo({ top: 60, behavior: "auto" }));
+    await expect(composer).not.toHaveAttribute("data-scroll-hidden", "true");
+
+    // Keyboard focus inside the composer reveals it while hidden.
+    await page.evaluate(() => window.scrollTo({ top: 240, behavior: "auto" }));
+    await expect(composer).toHaveAttribute("data-scroll-hidden", "true");
+    await composer.locator("input").focus();
+    await expect(composer).not.toHaveAttribute("data-scroll-hidden", "true");
+  });
+
   test("document summary opens at the top with cleaned bold formatting", async ({ page }) => {
     await page.setViewportSize({ width: 390, height: 820 });
     await mockDemoApi(page);