refactor(web): drop the dead shouldPinScroll helper and trim the reveal tests to two invariants

ChatPage no longer pins the page to (0, 0); keyboardRevealScrollDelta is the only caller-facing
helper left in keyboard-inset.ts. Update the isActive-effect comment that still described the pin.

web/src/lib/keyboard-inset.test.ts

@@ -3,7 +3,6 @@ import {
   computeKeyboardInset,
   keyboardRevealScrollDelta,
   KEYBOARD_INSET_MIN_PX,
-  shouldPinScroll,
 } from "./keyboard-inset";
 
 describe("computeKeyboardInset", () => {
@@ -69,23 +68,7 @@ describe("computeKeyboardInset", () => {
   });
 });
 
-describe("shouldPinScroll", () => {
-  it("pins while a keyboard inset is active", () => {
-    expect(shouldPinScroll(320)).toBe(true);
-  });
-
-  it("does not pin without a keyboard", () => {
-    expect(shouldPinScroll(0)).toBe(false);
-  });
-});
-
 describe("keyboardRevealScrollDelta", () => {
-  it("scrolls the page so the composer sits on the visual-viewport bottom", () => {
-    expect(
-      keyboardRevealScrollDelta(800, { height: 480, offsetTop: 0 }),
-    ).toBe(320);
-  });
-
   it("accounts for iOS visual-viewport offsetTop", () => {
     expect(
       keyboardRevealScrollDelta(800, { height: 480, offsetTop: 40 }),
@@ -97,10 +80,4 @@ describe("keyboardRevealScrollDelta", () => {
       keyboardRevealScrollDelta(480, { height: 480, offsetTop: 0 }),
     ).toBe(0);
   });
-
-  it("returns 0 for non-finite geometry", () => {
-    expect(
-      keyboardRevealScrollDelta(NaN, { height: 480, offsetTop: 0 }),
-    ).toBe(0);
-  });
 });

web/src/lib/keyboard-inset.ts

@@ -57,19 +57,6 @@ export function computeKeyboardInset(
   return inset >= KEYBOARD_INSET_MIN_PX ? inset : 0;
 }
 
-/**
- * Whether the page scroll should be pinned back to the top.
- *
- * The dashboard shell is a fixed `h-dvh` column and must never scroll, but
- * iOS Safari auto-scrolls the *page* when a focused input would sit under
- * the keyboard (xterm's hidden textarea triggers this). Pin whenever a
- * keyboard is present so the terminal chrome stays put; the terminal's own
- * scrollback handles content visibility.
- */
-export function shouldPinScroll(nextInsetPx: number): boolean {
-  return nextInsetPx > 0;
-}
-
 /** Pixels to `window.scrollBy` so the composer (xterm host bottom) sits on
  * the visible bottom of the visual viewport — just above the keyboard.
  * Pinning the page to (0, 0) fights iOS and can leave the input line off-screen. */

web/src/pages/ChatPage.tsx

@@ -1597,7 +1597,7 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
   // NS-434 follow-up: attach the visualViewport keyboard-inset listeners
   // ONLY while the chat tab is actually visible. ChatPage stays mounted
   // (display:none) on every other dashboard route, so unconditional
-  // listeners made the scroll pin (`window.scrollTo(0, 0)`) fire whenever a
+  // listeners made the composer reveal (`window.scrollBy`) fire whenever a
   // soft keyboard opened on Settings/Sessions/etc., fighting iOS Safari's
   // own scroll-into-view for the focused input there. The handlers read
   // through refs populated by the main PTY effect, so attach/detach here is