debug(desktop): on-screen scroll-metrics overlay (temporary)

apps/desktop/src/components/MessageList.tsx

@@ -2,11 +2,14 @@ import { useVirtualizer } from '@tanstack/react-virtual';
 import {
   forwardRef,
   useCallback,
+  useEffect,
   useImperativeHandle,
   useLayoutEffect,
   useRef,
   useState,
+  type MutableRefObject,
   type ReactNode,
+  type RefObject,
 } from 'react';
 
 import { isNearBottom, isNearTop } from '../lib/scrollController';
@@ -73,7 +76,13 @@ export const MessageList = forwardRef<MessageListHandle, MessageListProps>(funct
 
   const pinToBottom = useCallback(() => {
     const el = scrollElRef.current;
-    if (el) el.scrollTop = el.scrollHeight;
+    if (!el) return;
+    const before = el.scrollTop;
+    el.scrollTop = el.scrollHeight;
+    // eslint-disable-next-line no-console
+    console.log(
+      `[scroll] pin sh=${el.scrollHeight} ch=${el.clientHeight} top:${Math.round(before)}->${Math.round(el.scrollTop)}`,
+    );
   }, []);
 
   // Deferred reveal: when ready, position at the anchor (before paint), let one
@@ -90,6 +99,10 @@ export const MessageList = forwardRef<MessageListHandle, MessageListProps>(funct
     if (!ready || revealed || rows.length === 0) return;
     const el = scrollElRef.current;
     if (!el) return;
+    // eslint-disable-next-line no-console
+    console.log(
+      `[scroll] reveal anchor=${initialAnchor.type} rows=${rows.length} total=${Math.round(virtualizer.getTotalSize())} sh=${el.scrollHeight} ch=${el.clientHeight}`,
+    );
     if (initialAnchor.type === 'bottom') {
       pinToBottom();
       atBottomRef.current = true;
@@ -120,6 +133,10 @@ export const MessageList = forwardRef<MessageListHandle, MessageListProps>(funct
   // taller) and we were at the bottom, re-pin to the true bottom. Runs while
   // hidden too, so the list stays pinned through the initial measure settle.
   useLayoutEffect(() => {
+    // eslint-disable-next-line no-console
+    console.log(
+      `[scroll] stick? atBottom=${atBottomRef.current} total=${Math.round(virtualizer.getTotalSize())} revealed=${revealed}`,
+    );
     if (!atBottomRef.current) return;
     pinToBottom();
     // eslint-disable-next-line react-hooks/exhaustive-deps
@@ -172,6 +189,14 @@ export const MessageList = forwardRef<MessageListHandle, MessageListProps>(funct
   const items = virtualizer.getVirtualItems();
 
   return (
+    <>
+      <ScrollDebugOverlay
+        scrollElRef={scrollElRef}
+        getTotal={() => virtualizer.getTotalSize()}
+        atBottomRef={atBottomRef}
+        revealed={revealed}
+        ready={ready}
+      />
       <div
         ref={scrollElRef}
         onScroll={handleScroll}
@@ -199,5 +224,57 @@ export const MessageList = forwardRef<MessageListHandle, MessageListProps>(funct
         {/* 12px bottom breathing space (matches the old Virtuoso Footer). */}
         <div style={{ height: 12 }} />
       </div>
+    </>
   );
 });
+
+interface ScrollDebugOverlayProps {
+  scrollElRef: RefObject<HTMLDivElement>;
+  getTotal: () => number;
+  atBottomRef: MutableRefObject<boolean>;
+  revealed: boolean;
+  ready: boolean;
+}
+
+// TEMP on-screen debug readout — remove after the chat-switch scroll bug is fixed.
+function ScrollDebugOverlay({
+  scrollElRef,
+  getTotal,
+  atBottomRef,
+  revealed,
+  ready,
+}: ScrollDebugOverlayProps) {
+  const [, force] = useState(0);
+  useEffect(() => {
+    let raf = 0;
+    const loop = () => {
+      force((n) => (n + 1) % 1_000_000);
+      raf = requestAnimationFrame(loop);
+    };
+    raf = requestAnimationFrame(loop);
+    return () => cancelAnimationFrame(raf);
+  }, []);
+  const el = scrollElRef.current;
+  const sh = el?.scrollHeight ?? 0;
+  const ch = el?.clientHeight ?? 0;
+  const st = el ? Math.round(el.scrollTop) : 0;
+  return (
+    <div
+      style={{
+        position: 'fixed',
+        top: 8,
+        right: 8,
+        zIndex: 99999,
+        background: 'rgba(0,0,0,0.82)',
+        color: '#19ff8a',
+        font: '11px/1.4 monospace',
+        padding: '6px 9px',
+        borderRadius: 5,
+        pointerEvents: 'none',
+        whiteSpace: 'pre',
+      }}
+    >
+      {`scrollHeight=${sh}  clientHeight=${ch}\nscrollTop=${st}\ntotalSize=${Math.round(getTotal())}\nSCROLLABLE=${sh > ch + 4 ? 'YES' : 'NO'}\natBottom=${atBottomRef.current}  revealed=${revealed}  ready=${ready}`}
+    </div>
+  );
+}