@@ -47,7 +47,7 @@
     "to print the static chain that pulled a package into the closure."
   ],
   "maxBootBytes": 1723617,
-  "maxBootBrotliBytes": 479067,
+  "maxBootBrotliBytes": 429072,
   "forbiddenBootPackages": [
     "@pierre/diffs",
     "@pierre/theming",
 
@@ -108,6 +108,18 @@ function withElementClientHeight(
   }
 }
 
+// Waits out one scheduled rAF pass so "the pass ran and did nothing" is
+// distinguishable from "the pass has not run yet".
+async function flushScheduledViewportPass() {
+  await act(async () => {
+    await new Promise<void>((resolve) => {
+      window.requestAnimationFrame(() => {
+        window.requestAnimationFrame(() => resolve());
+      });
+    });
+  });
+}
+
 beforeEach(() => {
   vi.spyOn(window, "scrollTo").mockImplementation(() => {});
 });
@@ -310,6 +322,142 @@ describe("useMobileVisualViewportHeight", () => {
       expect(window.scrollTo).not.toHaveBeenCalled();
     });
   });
+
+  it("writes shell geometry only when a pass computes new values", async () => {
+    const visualViewport = new FakeVisualViewport();
+    visualViewport.offsetTop = 0;
+    await withFakeVisualViewport(visualViewport, async () => {
+      render(<VisualViewportShell enabled />);
+      const shell = screen.getByTestId("shell");
+      const shellHeightRoot = screen.getByTestId("shell-height-root");
+      expect(shell.style.height).toBe("500px");
+      const setShellHeightProperty = vi.spyOn(
+        shellHeightRoot.style,
+        "setProperty",
+      );
+
+      // Same geometry again: the pass must return before any style write, or
+      // every keyboard/URL-bar animation frame invalidates the whole tree.
+      act(() => {
+        visualViewport.dispatchEvent(new Event("resize"));
+      });
+      await flushScheduledViewportPass();
+      expect(setShellHeightProperty).not.toHaveBeenCalled();
+
+      act(() => {
+        visualViewport.height = 480;
+        visualViewport.dispatchEvent(new Event("resize"));
+      });
+      await waitFor(() => expect(shell.style.height).toBe("480px"));
+      expect(setShellHeightProperty).toHaveBeenCalledTimes(1);
+    });
+  });
+
+  it("reads the containing block only when the layout viewport can change", async () => {
+    const visualViewport = new FakeVisualViewport();
+    visualViewport.offsetTop = 0;
+    let containingBlockReads = 0;
+    await withElementClientHeight(
+      document.body,
+      () => {
+        containingBlockReads += 1;
+        return 800;
+      },
+      async () =>
+        withFakeVisualViewport(visualViewport, async () => {
+          render(<VisualViewportShell enabled />);
+          const shell = screen.getByTestId("shell");
+          expect(shell.style.height).toBe("500px");
+          const readsAfterMount = containingBlockReads;
+
+          // Visual-viewport ticks pan or resize only the visual viewport;
+          // they must reuse the cached containing-block height instead of
+          // forcing a full-document layout per animation frame.
+          act(() => {
+            visualViewport.offsetTop = 40;
+            visualViewport.dispatchEvent(new Event("scroll"));
+          });
+          await waitFor(() => expect(shell.style.top).toBe("40px"));
+          act(() => {
+            visualViewport.height = 460;
+            visualViewport.dispatchEvent(new Event("resize"));
+          });
+          await waitFor(() => expect(shell.style.height).toBe("460px"));
+          expect(containingBlockReads).toBe(readsAfterMount);
+
+          act(() => {
+            window.dispatchEvent(new Event("resize"));
+          });
+          await waitFor(() =>
+            expect(containingBlockReads).toBe(readsAfterMount + 1),
+          );
+        }),
+    );
+  });
+
+  it("runs a geometry pass when an editor is focused programmatically", async () => {
+    const visualViewport = new FakeVisualViewport();
+    visualViewport.offsetTop = 0;
+    await withElementClientHeight(
+      document.body,
+      () => 500,
+      async () =>
+        withFakeVisualViewport(visualViewport, async () => {
+          render(<VisualViewportShell enabled />);
+          const shell = screen.getByTestId("shell");
+          const editor = screen.getByTestId("editor");
+          // Native layout matches the visual viewport: no override applied.
+          expect(shell.style.height).toBe("");
+
+          // The keyboard shortens the visual viewport around the same time
+          // the composer autofocuses, without any window resize; the focus
+          // pass must pick the change up on its own.
+          visualViewport.height = 300;
+          act(() => editor.focus());
+          await waitFor(() => expect(shell.style.height).toBe("300px"));
+        }),
+    );
+  });
+
+  it("ignores visual viewport pans without a keyboard or an applied override", async () => {
+    const visualViewport = new FakeVisualViewport();
+    visualViewport.offsetTop = 0;
+    await withElementClientHeight(
+      document.body,
+      () => 500,
+      async () =>
+        withFakeVisualViewport(visualViewport, async () => {
+          render(<VisualViewportShell enabled />);
+          const shell = screen.getByTestId("shell");
+          const editor = screen.getByTestId("editor");
+          expect(shell.style.height).toBe("");
+
+          // A URL-bar pan with no keyboard: nothing to compensate.
+          act(() => {
+            visualViewport.offsetTop = 340;
+            visualViewport.dispatchEvent(new Event("scroll"));
+          });
+          await flushScheduledViewportPass();
+          expect(window.scrollTo).not.toHaveBeenCalled();
+          expect(shell.style.top).toBe("");
+
+          // With a keyboard editor focused, the same pan is Safari's
+          // focus-reveal pan and must still be compensated. Let the pan
+          // settle and the focus-scheduled pass run first, so that only the
+          // scroll handler's keyboard branch can produce the compensation.
+          visualViewport.offsetTop = 0;
+          act(() => editor.focus());
+          await flushScheduledViewportPass();
+          expect(shell.style.top).toBe("");
+          act(() => {
+            visualViewport.offsetTop = 340;
+            visualViewport.dispatchEvent(new Event("scroll"));
+          });
+          await waitFor(() => expect(shell.style.top).toBe("340px"));
+          expect(window.scrollTo).toHaveBeenCalledWith(0, 0);
+        }),
+    );
+  });
 });
 
 describe("shouldRestoreIOSViewportOnKeyboardDismissal", () => {
 
@@ -55,7 +55,22 @@ export function useMobileVisualViewportHeight(
     if (!shell || !shellHeightRoot || !enabled || !visualViewport) return;
 
     let animationFrame: number | null = null;
+    // The override last written to the shell, or null while none is applied.
+    // Writing shell `top`/`height` and the inherited `--bb-shell-height`
+    // invalidates computed style for the whole app tree, and passes run at
+    // visual-viewport event cadence (keyboard animation, URL-bar collapse),
+    // so a pass that recomputes unchanged geometry must not write at all.
+    let appliedOverride: { top: number; height: number } | null = null;
+    // Reading `document.body.clientHeight` forces a full-document layout. The
+    // shell's containing block only changes when the layout viewport does, so
+    // cache the read and mark it stale only on triggers that can resize the
+    // layout viewport — never on visualViewport ticks, which move or resize
+    // only the visual viewport.
+    let shellContainingBlockHeight = 0;
+    let shellContainingBlockHeightStale = true;
     const clearViewportOverride = () => {
+      if (appliedOverride === null) return;
+      appliedOverride = null;
       shell.style.removeProperty("top");
       shell.style.removeProperty("height");
       shellHeightRoot.style.removeProperty("--bb-shell-height");
@@ -68,12 +83,15 @@ export function useMobileVisualViewportHeight(
       }
 
       const visualViewportHeight = Math.round(visualViewport.height);
-      // `documentElement.clientHeight` is the visible viewport height for the
-      // root element, even when that root's actual CSS box extends behind an
-      // Android in-app browser toolbar. The body inherits the root box and
-      // therefore exposes the containing-block height the app shell really
-      // receives.
-      const shellContainingBlockHeight = document.body.clientHeight;
+      if (shellContainingBlockHeightStale) {
+        // `documentElement.clientHeight` is the visible viewport height for the
+        // root element, even when that root's actual CSS box extends behind an
+        // Android in-app browser toolbar. The body inherits the root box and
+        // therefore exposes the containing-block height the app shell really
+        // receives.
+        shellContainingBlockHeight = document.body.clientHeight;
+        shellContainingBlockHeightStale = false;
+      }
       const hasVisualViewportPan =
         visualViewport.offsetTop > 1 || window.scrollY > 0;
       if (
@@ -91,7 +109,16 @@ export function useMobileVisualViewportHeight(
         // compensation below also handles a visual-viewport-only pan.
         window.scrollTo(0, 0);
       }
-      shell.style.top = `${getVisualViewportPageTop(visualViewport)}px`;
+      const shellTop = getVisualViewportPageTop(visualViewport);
+      if (
+        appliedOverride !== null &&
+        appliedOverride.top === shellTop &&
+        appliedOverride.height === visualViewportHeight
+      ) {
+        return;
+      }
+      appliedOverride = { top: shellTop, height: visualViewportHeight };
+      shell.style.top = `${shellTop}px`;
       shell.style.height = `${visualViewportHeight}px`;
       // Fixed-position descendants cannot inherit the shell element's pixel
       // height. Publish the same correction through the existing shell-height
@@ -108,6 +135,28 @@ export function useMobileVisualViewportHeight(
       }
       animationFrame = window.requestAnimationFrame(updateHeight);
     };
+    // For triggers where the layout viewport may have changed: window resize,
+    // rotation, and an editor gaining focus — the pass that sizes the shell
+    // for the arriving keyboard must start from the real containing block,
+    // and these triggers are rare enough that the forced layout is fine.
+    const scheduleContainingBlockUpdate = () => {
+      shellContainingBlockHeightStale = true;
+      scheduleUpdate();
+    };
+    const handleVisualViewportScroll = () => {
+      // Keyboard-less visual-viewport pans (URL-bar collapse, momentum
+      // settling) don't change the containing block and need no override —
+      // the pan compensation exists for the keyboard focus-reveal pan. Only
+      // an already-applied override still has to track pans, because embedded
+      // browsers apply one without any keyboard.
+      if (
+        appliedOverride === null &&
+        !isKeyboardFocusTarget(document.activeElement)
+      ) {
+        return;
+      }
+      scheduleUpdate();
+    };
 
     // Safari with its bottom toolbar visible does not update the visual
     // viewport until the keyboard animation ends. Restore the normal shell
@@ -124,22 +173,29 @@ export function useMobileVisualViewportHeight(
     };
     const handleFocusIn = (event: FocusEvent) => {
       if (!isKeyboardFocusTarget(event.target)) return;
-      scheduleUpdate();
+      // Programmatic focus (composer autofocus) can be the only trigger for a
+      // keyboard, so this must always schedule a full, freshly measured pass.
+      scheduleContainingBlockUpdate();
     };
 
     updateHeight();
     visualViewport.addEventListener("resize", scheduleUpdate);
-    visualViewport.addEventListener("scroll", scheduleUpdate);
-    window.addEventListener("resize", scheduleUpdate);
+    visualViewport.addEventListener("scroll", handleVisualViewportScroll);
+    window.addEventListener("resize", scheduleContainingBlockUpdate);
+    window.addEventListener("orientationchange", scheduleContainingBlockUpdate);
     if (restoreImmediatelyOnKeyboardDismissal) {
       document.addEventListener("focusout", handleFocusOut);
       document.addEventListener("focusin", handleFocusIn);
     }
 
     return () => {
       visualViewport.removeEventListener("resize", scheduleUpdate);
-      visualViewport.removeEventListener("scroll", scheduleUpdate);
-      window.removeEventListener("resize", scheduleUpdate);
+      visualViewport.removeEventListener("scroll", handleVisualViewportScroll);
+      window.removeEventListener("resize", scheduleContainingBlockUpdate);
+      window.removeEventListener(
+        "orientationchange",
+        scheduleContainingBlockUpdate,
+      );
       if (restoreImmediatelyOnKeyboardDismissal) {
         document.removeEventListener("focusout", handleFocusOut);
         document.removeEventListener("focusin", handleFocusIn);
 
@@ -54,7 +54,10 @@ import {
 import { turnRequestLabel } from "@bb/client-core";
 import { splitStreamingMarkdown } from "./streaming-markdown-split.js";
 import { TurnRequestLabel } from "./TurnRequestLabel.js";
-import { MessageActionBar } from "./MessageActionBar.js";
+import {
+  MessageActionBar,
+  PROSE_COLUMN_INSET_CLASS,
+} from "./MessageActionBar.js";
 import {
   ConversationMessageOverflowToggle,
   useIsOverflowing,
@@ -678,7 +681,10 @@ function AssistantConversationMessage({
 
   return (
     <div
-      className="group/message w-full px-2 text-sm font-normal leading-relaxed"
+      className={cn(
+        "group/message w-full text-sm font-normal leading-relaxed",
+        PROSE_COLUMN_INSET_CLASS,
+      )}
       data-message-column=""
     >
       {/*