fix(hub): let the reader scroll up in the chat again

The chat could not be read back: any wheel gesture was undone in the frame
it happened in, and the "jump to latest" button never appeared.

None of the pins in ChatPanel are at fault -- every one of them is guarded
by "only if the reader is at the bottom". The reader never got to stop
being at the bottom.

fit() set the panel's height, read documentElement.scrollHeight back and
subtracted the overflow, so the document alternately did and did not
overflow the window. The page scrollbar appeared and vanished with it and
visualViewport fired resize at every pass -- the event fit() is bound to.
It therefore re-entered itself for the life of the panel: measured at 240
firings in two seconds on a page nobody was touching, against 2 for a bare
document. Each pass ran fitAndPin, which re-pinned the list to the bottom
before the scroll event that would have recorded the gesture was delivered
a frame later, so atBottomRef never went false.

- fit() learns the space below the panel once and remembers it on the
  element instead of re-deriving it by writing and measuring back. At the
  steady state it writes nothing, so it produces no resize. A real window
  resize or an orientation change forgets the learnt value and measures
  again (the page under the panel may have reflowed); visualViewport
  deliberately does not, since a phone fires it constantly.

- The scroll-to-bottom is now scoped to *arrival*, which is all it was ever
  for: opening the group, or coming back to the Chat tab, including the
  thumbnails and link-preview cards that keep growing the list for a second
  afterwards. It ends when the reader takes hold of the scroll, and the
  ResizeObserver disconnects there.

- That release is recorded from the gesture (wheel/touchmove/pointerdown/
  keydown), not from the scroll event, which arrives too late to protect
  anything.

Unchanged: landing on the newest message, following new messages while
already at the bottom, the "load older" anchor and the unread marker.

tests/harness/chat_scroll_probe.py mounts the real ChatPanel in a browser
and reads a conversation back; test_chat_scroll_up.py asserts against it.
With the fix reverted, five of its six tests fail and the sixth -- landing
on the newest message -- still passes, which is the property that must not
have been traded away. A structural test cannot see any of this, which is
why it is measured.

test_layout_responsive.py pinned the listener's name and follows the
rename.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8oRqEHhnKUr1NfmTVdcyL

1 files changed, 81 insertions, 18 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/static/chat-app.js b/packages/meshbay-hub/src/meshbay_hub/static/chat-app.js
index 1850c74..3bc110f 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/chat-app.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/chat-app.js
@@ -216,6 +216,12 @@ function ChatPanel({ transportRef, username, entries, gekRef, onRefreshIndex,
   // them but before the browser paints.
   const anchorRef = useRef(null);
   const atBottomRef = useRef(true);
+  // Landing on the newest message is an *arrival* behaviour: it belongs to
+  // opening the group and to coming back to the Chat tab, and it has to survive
+  // the thumbnails and link-preview cards that keep growing the list for a
+  // second or two afterwards. It ends the moment the reader asks to move, and
+  // after that nothing may move the view for them.
+  const arrivingRef = useRef(true);
 
   useEffect(() => {
     if (status !== 'connected') return;
@@ -230,7 +236,13 @@ function ChatPanel({ transportRef, username, entries, gekRef, onRefreshIndex,
           setMessages(msgs);
           requestAnimationFrame(() => {
             const l = listRef.current;
-            if (l) { l.scrollTop = l.scrollHeight; atBottomRef.current = true; }
+            // Not if the reader already moved: a slow node means this page
+            // lands seconds after the panel opened, and by then they may be
+            // reading somewhere else entirely.
+            if (l && arrivingRef.current) {
+              l.scrollTop = l.scrollHeight;
+              atBottomRef.current = true;
+            }
           });
         })
         .catch(() => { loadedRef.current = false; });
@@ -284,27 +296,52 @@ function ChatPanel({ transportRef, username, entries, gekRef, onRefreshIndex,
     if (atBottomRef.current) list.scrollTop = list.scrollHeight;
   }, [messages, hasMore]);
 
-  // Keep the view pinned to the newest message while the reader is at the
-  // bottom, through everything that grows the content *after* the initial
-  // paint: attachment thumbnails and link-preview cards fetched over the
-  // network, late layout, and the panel resizing itself with `fit()` below.
-  // Without this, opening the Chat tab reliably lands a screen or two above
-  // the last message — the layout effect above ran when the list was still
-  // short. Does nothing once the reader scrolls up (`atBottomRef` is false),
-  // so it never fights reading back or the "load older" anchor.
+  // Hold the arrival at the newest message through everything that grows the
+  // content *after* the initial paint: attachment thumbnails and link-preview
+  // cards fetched over the network, late layout, and the panel resizing itself
+  // with `fit()` below. Without this, opening the Chat tab reliably lands a
+  // screen or two above the last message — the layout effect above ran when
+  // the list was still short.
+  //
+  // Bounded by `arrivingRef`, not by `atBottomRef` alone. The at-bottom test
+  // has a 40px tolerance and is fed by an event delivered a frame late, so on
+  // its own it let a resize storm hold the reader against the end of the
+  // conversation with no way back up it. Once the reader has asked to move,
+  // this stops observing entirely.
   useEffect(() => {
     const list = listRef.current;
-    if (!list || typeof ResizeObserver === 'undefined') return;
+    if (!list || typeof ResizeObserver === 'undefined' || !arrivingRef.current) return;
+    // Declared before `stick` closes over it: a `const` further down would be
+    // in its temporal dead zone here, which is the hook-ordering trap this
+    // codebase has already paid for once.
+    let ro = null;
     const stick = () => {
+      if (!arrivingRef.current) { if (ro) ro.disconnect(); return; }
       if (atBottomRef.current) list.scrollTop = list.scrollHeight;
     };
-    const ro = new ResizeObserver(stick);
+    ro = new ResizeObserver(stick);
     ro.observe(list);
     for (const child of list.children) ro.observe(child);
     stick();
     return () => ro.disconnect();
   }, [messages]);
 
+  // The reader taking hold of the scroll ends the arrival, and it has to be
+  // recorded here rather than in `onScroll`: a `scroll` event is delivered at
+  // the next rendering step, so anything that pins in between undoes the
+  // movement before the handler that would have stopped it ever runs. These
+  // fire with the gesture itself.
+  useEffect(() => {
+    const list = listRef.current;
+    if (!list) return;
+    const release = () => { arrivingRef.current = false; };
+    const events = ['wheel', 'touchmove', 'pointerdown', 'keydown'];
+    for (const name of events) list.addEventListener(name, release, { passive: true });
+    return () => {
+      for (const name of events) list.removeEventListener(name, release, { passive: true });
+    };
+  }, []);
+
   // Entering the Chat tab should leave the cursor in the composer, ready to
   // type — the panel mounts fresh on every tab switch, so a mount effect is
   // the tab-entry hook. `preventScroll` because the layout effects above are
@@ -332,7 +369,6 @@ function ChatPanel({ transportRef, username, entries, gekRef, onRefreshIndex,
       // Document-relative, so a page that happens to be scrolled does not skew
       // the result — the answer must be the same either way.
       const top = el.getBoundingClientRect().top + window.scrollY;
-      el.style.height = `${Math.max(CHAT_MIN_HEIGHT, vh - top - CHAT_BOTTOM_GAP)}px`;
       // What sits *below* the panel is not knowable from up here — today it is
       // `.main`'s 24px bottom padding against this 16px gap, which left the
       // document 8px taller than the window and a scrollbar on the chat tab at
@@ -340,10 +376,28 @@ function ChatPanel({ transportRef, username, entries, gekRef, onRefreshIndex,
       // change to the page break it again, the leftover is measured and taken
       // off. Self-correcting: anything added under the panel is absorbed the
       // same way.
+      //
+      // Remembered on the element rather than re-derived on every call, because
+      // this runs on `resize` and so can *cause* the event it listens for.
+      // Setting the naive height, reading the overflow back and subtracting it
+      // means the document alternately does and does not overflow the window: a
+      // page scrollbar appears and vanishes with it, `visualViewport` fires
+      // `resize` at every pass, and fit() re-enters itself for the life of the
+      // panel. Measured 2026-09-01 on a page nobody was touching: 240 firings
+      // in two seconds, against 2 for a bare document. Each one re-pinned the
+      // list to the bottom, so every attempt to scroll up was undone inside the
+      // same frame — before the `scroll` event that would have recorded it was
+      // even delivered, which is why the reader could not move and the "jump to
+      // latest" button never appeared. Converged, this writes nothing.
+      const below = el._chatFitBelow || CHAT_BOTTOM_GAP;
+      const target = Math.max(CHAT_MIN_HEIGHT, vh - top - below);
+      if (Math.abs(target - el.getBoundingClientRect().height) > 0.5) {
+        el.style.height = `${target}px`;
+      }
       const over = document.documentElement.scrollHeight - vh;
       if (over > 0) {
-        el.style.height =
-          `${Math.max(CHAT_MIN_HEIGHT, el.getBoundingClientRect().height - over)}px`;
+        el._chatFitBelow = below + over;
+        el.style.height = `${Math.max(CHAT_MIN_HEIGHT, target - over)}px`;
       }
     };
     const fitAndPin = () => {
@@ -351,13 +405,19 @@ function ChatPanel({ transportRef, username, entries, gekRef, onRefreshIndex,
       const l = listRef.current;
       if (l && atBottomRef.current) l.scrollTop = l.scrollHeight;
     };
+    // A real viewport change can also mean the page below the panel reflowed,
+    // so the learnt leftover is dropped and measured again. Not on
+    // `visualViewport`: an Android keyboard changes the viewport, not the
+    // layout under the panel, and re-learning there would put the oscillation
+    // back on the one platform that fires that event constantly.
+    const refit = () => { el._chatFitBelow = 0; fitAndPin(); };
     fitAndPin();
-    window.addEventListener('resize', fitAndPin);
-    window.addEventListener('orientationchange', fitAndPin);
+    window.addEventListener('resize', refit);
+    window.addEventListener('orientationchange', refit);
     window.visualViewport?.addEventListener('resize', fitAndPin);
     return () => {
-      window.removeEventListener('resize', fitAndPin);
-      window.removeEventListener('orientationchange', fitAndPin);
+      window.removeEventListener('resize', refit);
+      window.removeEventListener('orientationchange', refit);
       window.visualViewport?.removeEventListener('resize', fitAndPin);
     };
   }, []);
@@ -366,6 +426,9 @@ function ChatPanel({ transportRef, username, entries, gekRef, onRefreshIndex,
     const el = e.target;
     const bottom = el.scrollHeight - el.scrollTop - el.clientHeight < 40;
     atBottomRef.current = bottom;
+    // Belt and braces for a scroll no gesture listener saw — a scrollbar
+    // dragged from outside the list, a "find in page" jump, assistive tech.
+    if (!bottom) arrivingRef.current = false;
     setAtBottom(bottom);
     if (bottom) setUnreadFrom(null);
   }, []);