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, 102 insertions, 0 deletions

diff --git a/packages/meshbay-hub/tests/test_chat_scroll_up.py b/packages/meshbay-hub/tests/test_chat_scroll_up.py
new file mode 100644
index 0000000..6cafcdc
--- /dev/null
+++ b/packages/meshbay-hub/tests/test_chat_scroll_up.py
@@ -0,0 +1,102 @@
+"""
+Reading back through the conversation must work.
+
+The scroll-to-bottom on arrival has been patched five times, and the sixth
+patch took the other side away: the panel re-pinned the list about 120 times a
+second, so a wheel gesture was undone in the frame it happened in and the older
+messages became unreachable. Every pin in the source is guarded by "only if the
+reader is at the bottom" — the reader simply never got to stop being at the
+bottom, because the `scroll` event that records it is delivered a frame after
+the pin that erased it.
+
+None of that is visible in the source, which is why this measures the real
+`ChatPanel` in a browser instead of reading `chat-app.js`. `test_chat_scroll_
+bottom.py` keeps the structural guards; this one keeps the behaviour.
+"""
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+HARNESS = Path(__file__).parent / "harness" / "chat_scroll_probe.py"
+STATIC = Path(__file__).resolve().parents[1] / "src" / "meshbay_hub" / "static"
+
+pytestmark = pytest.mark.skipif(
+    shutil.which("google-chrome") is None or not (STATIC / "chat-app.js").exists(),
+    reason="Chrome or the SPA sources are not available")
+
+# A bare document fires a couple of these as it settles. Anything above this is
+# the panel driving itself, which is what the loop looked like.
+IDLE_RESIZE_CEILING = 20
+
+# The harness scrolls up by six frames of 120px. Chrome's scroll anchoring
+# moves the reader with the content when preview cards land above them, so the
+# distance from the bottom is not expected to be exactly 720 afterwards — only
+# to stay well clear of it.
+SCROLLED_UP_FLOOR = 400
+
+
+@pytest.fixture(scope="module")
+def probe():
+    run = subprocess.run(["python3", str(HARNESS)], capture_output=True, timeout=180)
+    assert run.returncode == 0, run.stderr.decode()[-2000:]
+    data = json.loads(run.stdout.decode())
+    return data, {s["label"]: s for s in data["steps"]}
+
+
+def test_arrives_on_the_newest_message(probe):
+    """Opening the tab lands at the end of the conversation, after the late
+    layout and the panel sizing itself."""
+    _, steps = probe
+    assert steps["arrived"]["fromBottom"] < 40, (
+        "the chat must open on the newest message; it opened "
+        f"{steps['arrived']['fromBottom']}px above it")
+
+
+def test_the_panel_does_not_resize_itself(probe):
+    """fit() runs on `resize` and must not produce one. When it did, it re-ran
+    every frame and re-pinned the scroll with it."""
+    data, _ = probe
+    assert data["idle"]["viewportResizes"] <= IDLE_RESIZE_CEILING, (
+        f"{data['idle']['viewportResizes']} viewport resizes on a page nobody "
+        "touched -- fit() is feeding the event it listens for, and every pass "
+        "re-pins the chat to the bottom")
+    assert data["idle"]["documentOverflow"] <= 0, (
+        "the chat tab must not leave the document taller than the window")
+
+
+def test_scrolling_up_holds(probe):
+    """The gesture must survive the frame it happened in."""
+    _, steps = probe
+    assert steps["scrolled up"]["fromBottom"] >= SCROLLED_UP_FLOOR, (
+        "scrolling up was undone: the list came back to "
+        f"{steps['scrolled up']['fromBottom']}px from the bottom")
+    assert steps["scrolled up"]["jumpButton"], (
+        "the panel never noticed the reader leave the bottom -- the pin beat "
+        "the scroll event, so atBottom stayed true and no jump button appeared")
+
+
+def test_late_content_does_not_drag_the_reader_down(probe):
+    """Link previews and thumbnails arrive over the following seconds and grow
+    the list. That is what the arrival pin is for, and it must be over by
+    now."""
+    _, steps = probe
+    assert steps["previews landed"]["fromBottom"] >= SCROLLED_UP_FLOOR, (
+        "preview cards landing pulled the reader back to the bottom")
+
+
+def test_a_new_message_does_not_yank_the_reader_down(probe):
+    """Somebody writing while you read back marks the spot; it does not move
+    you."""
+    _, steps = probe
+    assert steps["message arrived"]["fromBottom"] >= SCROLLED_UP_FLOOR, (
+        "an incoming message pulled the reader away from what they were reading")
+
+
+def test_a_resize_does_not_yank_the_reader_down(probe):
+    """A window resize, a rotation, or a phone's URL bar collapsing."""
+    _, steps = probe
+    assert steps["window resized"]["fromBottom"] >= SCROLLED_UP_FLOOR, (
+        "a resize pulled the reader back to the bottom")