diff --git a/canvas/src/components/tabs/chat/AttachmentImage.tsx b/canvas/src/components/tabs/chat/AttachmentImage.tsx
index a123856fb..79c8c7c1c 100644
--- a/canvas/src/components/tabs/chat/AttachmentImage.tsx
+++ b/canvas/src/components/tabs/chat/AttachmentImage.tsx
@@ -166,11 +166,12 @@ export function AttachmentImage({ workspaceId, attachment, onDownload, tone }: P
         open={open}
         onClose={() => setOpen(false)}
         ariaLabel={`Preview of ${attachment.name}`}
+        contained
       >
         <img
           src={state.blobUrl}
           alt={attachment.name}
-          className="max-w-[95vw] max-h-[90vh] object-contain"
+          className="max-w-full max-h-full object-contain"
         />
       </AttachmentLightbox>
     </>
diff --git a/canvas/src/components/tabs/chat/AttachmentLightbox.tsx b/canvas/src/components/tabs/chat/AttachmentLightbox.tsx
index 09f4cb016..f52c0bc87 100644
--- a/canvas/src/components/tabs/chat/AttachmentLightbox.tsx
+++ b/canvas/src/components/tabs/chat/AttachmentLightbox.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-// AttachmentLightbox — shared fullscreen modal for image / PDF /
+// AttachmentLightbox — shared modal for image / PDF /
 // (future) any-fullscreen-renderable kind. Owns:
 //   - Backdrop + centered viewport
 //   - Esc to close
@@ -14,11 +14,11 @@
 //
 // Design choices:
 //
-// 1. Portals — we don't use ReactDOM.createPortal because the canvas
-//    chat surface already renders at a high z-index and the modal's
-//    fixed-position layout reaches the viewport regardless. Saves a
-//    portal mount in the common case + avoids the SSR warning (canvas
-//    is "use client" but the parent shell is server-rendered).
+// 1. Portals — we don't use ReactDOM.createPortal because the chat tab
+//    already gives us a positioned container and the preview should stay
+//    inside that panel. Saves a portal mount in the common case + avoids
+//    the SSR warning (canvas is "use client" but the parent shell is
+//    server-rendered).
 //
 // 2. Focus trap — inline implementation (not a 3rd-party dep). The
 //    chat lightbox needs to trap focus only across two interactive
@@ -41,13 +41,17 @@ interface Props {
    *  the dialog opens. The caller knows what's inside (image alt
    *  text, PDF filename) and supplies it. */
   ariaLabel: string;
+  /** Constrain the preview to the nearest positioned ancestor instead
+   *  of the whole browser viewport. ChatTab passes this so previews
+   *  stay inside the active side-panel tab. */
+  contained?: boolean;
   /** The thing being shown in fullscreen — <img>, <embed>, etc.
    *  Caller is responsible for sizing it to fit the viewport (we
    *  give it max-w-full max-h-full via CSS). */
   children: ReactNode;
 }
 
-export function AttachmentLightbox({ open, onClose, ariaLabel, children }: Props) {
+export function AttachmentLightbox({ open, onClose, ariaLabel, contained = false, children }: Props) {
   const closeButtonRef = useRef<HTMLButtonElement>(null);
   const previousFocusRef = useRef<HTMLElement | null>(null);
 
@@ -90,12 +94,19 @@ export function AttachmentLightbox({ open, onClose, ariaLabel, children }: Props
 
   if (!open) return null;
 
+  const rootClass = contained
+    ? "absolute inset-0 z-50 flex items-center justify-center bg-black/85 motion-reduce:transition-none transition-opacity"
+    : "fixed inset-0 z-50 flex items-center justify-center bg-black/85 motion-reduce:transition-none transition-opacity";
+  const contentClass = contained
+    ? "h-full w-full p-3 flex items-center justify-center"
+    : "max-w-[95vw] max-h-[90vh] flex items-center justify-center";
+
   return (
     <div
       role="dialog"
       aria-modal="true"
       aria-label={ariaLabel}
-      className="fixed inset-0 z-50 flex items-center justify-center bg-black/85 motion-reduce:transition-none transition-opacity"
+      className={rootClass}
       onClick={onBackdropClick}
     >
       {/* Close button — top-right, large hit area, keyboard-focusable.
@@ -112,7 +123,7 @@ export function AttachmentLightbox({ open, onClose, ariaLabel, children }: Props
         </svg>
       </button>
       <div
-        className="max-w-[95vw] max-h-[90vh] flex items-center justify-center"
+        className={contentClass}
         onClick={(e) => e.stopPropagation()}
       >
         {children}
diff --git a/canvas/src/components/tabs/chat/AttachmentPDF.tsx b/canvas/src/components/tabs/chat/AttachmentPDF.tsx
index 8b519549a..9093ca691 100644
--- a/canvas/src/components/tabs/chat/AttachmentPDF.tsx
+++ b/canvas/src/components/tabs/chat/AttachmentPDF.tsx
@@ -19,8 +19,8 @@
 // suppress the toolbar; we keep it on so the user gets standard
 // PDF affordances.
 //
-// Fullscreen: AttachmentLightbox hosts the PDF at viewport size on
-// click. Same shared modal as image — third caller justifies the
+// Preview: AttachmentLightbox hosts the PDF inside the active chat tab
+// on click. Same shared modal as image — third caller justifies the
 // abstraction (per RFC #2991 design).
 //
 // Failure modes:
@@ -144,8 +144,9 @@ export function AttachmentPDF({ workspaceId, attachment, onDownload, tone }: Pro
         open={open}
         onClose={() => setOpen(false)}
         ariaLabel={`Preview of ${attachment.name}`}
+        contained
       >
-        <div className="h-[90vh] w-[95vw] overflow-hidden rounded-lg border border-white/20 bg-white shadow-2xl">
+        <div className="h-full w-full overflow-hidden rounded-lg border border-white/20 bg-white shadow-2xl">
           <iframe
             src={`${state.blobUrl}#view=FitH`}
             title={attachment.name}
diff --git a/canvas/src/components/tabs/chat/__tests__/AttachmentLightbox.test.tsx b/canvas/src/components/tabs/chat/__tests__/AttachmentLightbox.test.tsx
index 50bb25073..76ef0234e 100644
--- a/canvas/src/components/tabs/chat/__tests__/AttachmentLightbox.test.tsx
+++ b/canvas/src/components/tabs/chat/__tests__/AttachmentLightbox.test.tsx
@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 /**
- * AttachmentLightbox — fullscreen modal for image / PDF preview.
+ * AttachmentLightbox — modal for image / PDF preview.
  *
  * Owns: backdrop + viewport, Esc to close, click-outside to close,
  * focus trap (close button focus on open, restore on close),
@@ -135,6 +135,22 @@ describe("AttachmentLightbox — render", () => {
     const closeBtn = document.querySelector('button[aria-label="Close preview"]');
     expect(closeBtn).toBeTruthy();
   });
+
+  it("uses absolute positioning when contained=true", () => {
+    render(
+      <AttachmentLightbox
+        open={true}
+        onClose={vi.fn()}
+        ariaLabel="Preview"
+        contained
+      >
+        <MockContent />
+      </AttachmentLightbox>,
+    );
+    const dialog = document.querySelector('[role="dialog"]');
+    expect(dialog?.className).toContain("absolute");
+    expect(dialog?.className).not.toContain("fixed");
+  });
 });
 
 // ─── Focus management ─────────────────────────────────────────────────────────
diff --git a/canvas/src/components/tabs/chat/__tests__/AttachmentPDF.test.tsx b/canvas/src/components/tabs/chat/__tests__/AttachmentPDF.test.tsx
index 763acf896..a8a9aec3a 100644
--- a/canvas/src/components/tabs/chat/__tests__/AttachmentPDF.test.tsx
+++ b/canvas/src/components/tabs/chat/__tests__/AttachmentPDF.test.tsx
@@ -1,12 +1,12 @@
 // @vitest-environment jsdom
 /**
- * AttachmentPDF — inline PDF preview button + click-to-fullscreen lightbox.
+ * AttachmentPDF — inline PDF preview button + click-to-panel lightbox.
  *
  * Per RFC #2991 PR-3: platform-auth URIs fetch bytes → Blob → ObjectURL;
  * external URIs use the raw URL directly. State machine: idle → loading →
  * ready/error. Loading skeleton shown while fetching. Error falls back to
  * AttachmentChip. Clicking the preview button opens AttachmentLightbox with
- * <embed>. Blob URL cleaned up on unmount.
+ * a browser PDF iframe. Blob URL cleaned up on unmount.
  *
  * NOTE: No @testing-library/jest-dom import — use DOM APIs for assertions.
  *
@@ -158,10 +158,12 @@ describe("AttachmentPDF — ready", () => {
     });
     const dialog = document.querySelector('[role="dialog"]');
     expect(dialog?.getAttribute("aria-label")).toContain("report.pdf");
+    expect(dialog?.className).toContain("absolute");
     const frame = dialog?.querySelector("iframe") as HTMLIFrameElement | null;
     expect(frame).toBeTruthy();
     expect(frame?.getAttribute("title")).toBe("report.pdf");
     expect(frame?.className).toContain("bg-white");
+    expect(frame?.parentElement?.className).toContain("w-full");
     expect(dialog?.querySelector("embed")).toBeNull();
   });
 
diff --git a/canvas/src/components/tabs/chat/__tests__/AttachmentPreview.test.tsx b/canvas/src/components/tabs/chat/__tests__/AttachmentPreview.test.tsx
index 0b7ee0ebd..e104d350d 100644
--- a/canvas/src/components/tabs/chat/__tests__/AttachmentPreview.test.tsx
+++ b/canvas/src/components/tabs/chat/__tests__/AttachmentPreview.test.tsx
@@ -237,11 +237,13 @@ describe("AttachmentPreview dispatch", () => {
       expect(screen.getByLabelText(/Open doc\.pdf preview/i)).toBeTruthy();
     });
 
-    // Click → lightbox opens with <embed> inside.
+    // Click → panel-contained lightbox opens with a browser PDF iframe.
     fireEvent.click(screen.getByLabelText(/Open doc\.pdf preview/i));
     const dialog = await screen.findByRole("dialog");
     expect(dialog).toBeTruthy();
-    expect(dialog.querySelector("embed[type='application/pdf']")).not.toBeNull();
+    expect(dialog.className).toContain("absolute");
+    expect(dialog.querySelector("iframe")).not.toBeNull();
+    expect(dialog.querySelector("embed")).toBeNull();
   });
 
   it("kind=pdf fetch fails → falls back to chip", async () => {