[eric] chat: fullscreen side gutters scroll the transcript instead of eating the wheel

(cherry picked from commit fe1ebaa5 on eric/feature-requests)
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PNA9xhjw5SkfuB2tJ5KwCj

frontend/src/app/pages/AgentChat/AgentChat.tsx

@@ -84,6 +84,7 @@ import { shallowEqual } from 'react-redux';
 import { useClaudeTokens, useThemeMode } from '@/shared/styles/ThemeContext';
 import { parseMcpToolName, getMcpInputSummary } from '@/shared/mcpToolMeta';
 import { isNarration } from './parsing/isNarration';
+import { shouldForwardGutterWheel } from './gutterWheel';
 import { openMarketplace } from '@/app/pages/Directory/openMarketplace';
 
 const CONTEXT_WINDOWS: Record<string, number> = {
@@ -831,6 +832,23 @@ const AgentChat: React.FC<AgentChatProps> = ({ sessionId: sessionIdProp, onClose
     };
   }, []);
 
+  // Fullscreen centers the column inside wide empty gutters, and the gutter is where the cursor naturally rests; a vertical wheel there can only mean "scroll the thread". Only the BARE gutter forwards (target === currentTarget), so nothing inside the column can be double-scrolled, honoring the one-owner rule.
+  const onGutterWheel = useCallback((e: React.WheelEvent<HTMLDivElement>) => {
+    if (!shouldForwardGutterWheel({
+      ctrlKey: e.ctrlKey, metaKey: e.metaKey, deltaX: e.deltaX, deltaY: e.deltaY,
+      targetIsGutter: e.target === e.currentTarget,
+    })) return;
+    const el = scrollContainerRef.current;
+    if (!el) return;
+    if (e.deltaY < 0) {
+      // Mirror the container's own wheel: reading up from the gutter is real follow-away intent.
+      userScrollIntentUntilRef.current = performance.now() + USER_SCROLL_INTENT_MS;
+      pinAbortRef.current = true;
+    }
+    el.scrollTop += e.deltaY;
+    e.stopPropagation();
+  }, []);
+
   const scrollToBottomRafRef = useRef<number | null>(null);
   const scrollToBottom = useCallback(() => {
     const el = scrollContainerRef.current;
@@ -1638,7 +1656,10 @@ const AgentChat: React.FC<AgentChatProps> = ({ sessionId: sessionIdProp, onClose
   const statusStyle = STATUS_STYLES[session.status] || { color: c.text.tertiary, bg: c.bg.secondary };
 
   return (
-    <Box sx={{ display: 'flex', height: '100%', ...(fullscreenWash && { background: fullscreenWash }) }}>
+    <Box
+      onWheel={fullscreenChat ? onGutterWheel : undefined}
+      sx={{ display: 'flex', height: '100%', ...(fullscreenWash && { background: fullscreenWash }) }}
+    >
       <ContextDrawer />
       {/* Marks transcript AND composer as one chat, so Cmd+A from the composer can still find the conversation (ENG-231). */}
       <Box data-chat-root sx={{ display: 'flex', flexDirection: 'column', flex: 1, minWidth: 0, overflow: 'hidden', ...(fullscreenChat && { maxWidth: FULLSCREEN_READING_MAX_W, width: '100%', mx: 'auto' }) }}>

frontend/src/app/pages/AgentChat/gutterWheel.test.ts

@@ -0,0 +1,24 @@
+// Run: npm test (frontend/scripts/run-tests.mjs)
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { shouldForwardGutterWheel } from './gutterWheel.ts';
+
+const base = { ctrlKey: false, metaKey: false, deltaX: 0, deltaY: 40, targetIsGutter: true };
+
+test('a plain vertical wheel on the bare gutter forwards', () => {
+  assert.equal(shouldForwardGutterWheel(base), true);
+  assert.equal(shouldForwardGutterWheel({ ...base, deltaY: -40 }), true);
+});
+
+test('anything inside the column keeps its own scroll (no double-scroll)', () => {
+  assert.equal(shouldForwardGutterWheel({ ...base, targetIsGutter: false }), false);
+});
+
+test('pinch-zoom stays free on every surface', () => {
+  assert.equal(shouldForwardGutterWheel({ ...base, ctrlKey: true }), false);
+  assert.equal(shouldForwardGutterWheel({ ...base, metaKey: true }), false);
+});
+
+test('horizontal-dominant gestures pass through to the canvas', () => {
+  assert.equal(shouldForwardGutterWheel({ ...base, deltaX: 80, deltaY: 10 }), false);
+});

frontend/src/app/pages/AgentChat/gutterWheel.ts

@@ -0,0 +1,13 @@
+// Haik: in fullscreen, scrolling only works over the center text column; the wide side gutters eat
+// the wheel. A wheel on the BARE gutter forwards to the transcript; everything else keeps its owner.
+export function shouldForwardGutterWheel(e: {
+  ctrlKey: boolean;
+  metaKey: boolean;
+  deltaX: number;
+  deltaY: number;
+  targetIsGutter: boolean;
+}): boolean {
+  if (e.ctrlKey || e.metaKey) return false;
+  if (Math.abs(e.deltaX) > Math.abs(e.deltaY)) return false;
+  return e.targetIsGutter;
+}