fix(scroll): do not unpin a pinned tail reader on an above-tail collapse clamp (#7605)

tests/test_issue1731_upward_scroll_unpins.py

@@ -71,7 +71,10 @@ def test_scroll_listener_detects_upward_motion():
 def test_upward_scroll_unpins_immediately_without_hysteresis():
     """Upward motion sets _scrollPinned=false and resets the counter, no count needed."""
     block = _scroll_listener_block()
-    if_idx = block.index("if(movedUp)")
+    # The upward branch is the clamped one: it only fires once the reader has left
+    # the true bottom (bottomDistance>1), so an above-tail collapse clamp cannot
+    # unpin a reader who is still flush at the tail.
+    if_idx = block.index("if(movedUp&&bottomDistance>1){")
     else_idx = block.find("else", if_idx)
     assert else_idx > if_idx, "upward / downward branches not found (#1731)"
     upward_branch = block[if_idx:else_idx]