test: pin #3250 scroll-intent window + fix #2823 openFile anchor for #3262

- Pin MESSAGE_UPWARD_INTENT_MS>=2000ms + intent-helper-uses-constant +
  downward-repin-independence (#3250, co-authored emanon312).
- Update the #2823 large-markdown-preview test's openFile() signature
  anchor for #3262's openFile(path, opts={}) extension.

Co-authored-by: emanon312 <emanon312@users.noreply.github.com>

tests/test_issue2823_large_markdown_preview.py

@@ -7,7 +7,7 @@ WORKSPACE_JS = Path("static/workspace.js").read_text(encoding="utf-8")
 
 
 def _open_file_block() -> str:
-    marker = "async function openFile(path){"
+    marker = "async function openFile(path, opts={}){"
     start = WORKSPACE_JS.find(marker)
     assert start != -1, "openFile() not found in workspace.js"
     end = WORKSPACE_JS.find("\nfunction downloadFile", start)

tests/test_issue3250_upward_scroll_intent_window.py

@@ -0,0 +1,96 @@
+"""Regression test for #3250: upward-scroll intent window during streaming.
+
+The pre-fix `MESSAGE_UPWARD_INTENT_MS` window was only 450ms. When a user
+scrolled up to read earlier content during a streaming response and then
+*paused* to read (>450ms since their last wheel/touch event), the intent
+expired. Subsequent DOM-layout changes from the streaming markdown parser
+(smd), tool-card insertions, or code re-highlighting then produced scroll
+events that `_recentMessageUpwardIntent()` no longer attributed to the user
+(`movedUp = false`). If the resulting position sat inside the 250px
+near-bottom zone for two consecutive samples, `_scrollPinned` flipped back to
+true and the next streaming token snapped the user to the bottom.
+
+The fix widens the window to 2000ms so a brief reading pause no longer drops
+the user's intent. Direction detection is unchanged — downward motion still
+re-pins regardless of the timeout because `movedUp` additionally requires
+`top < _lastScrollTop - 2` — so this is a pure intent-duration tuning, not a
+relaxation of the re-pin semantics.
+"""
+
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parents[1]
+UI_JS = (REPO / "static" / "ui.js").read_text(encoding="utf-8")
+
+
+def _intent_window_ms() -> int:
+    """Extract the numeric value of the MESSAGE_UPWARD_INTENT_MS constant."""
+    marker = "const MESSAGE_UPWARD_INTENT_MS="
+    idx = UI_JS.find(marker)
+    assert idx != -1, "MESSAGE_UPWARD_INTENT_MS constant not found in ui.js"
+    start = idx + len(marker)
+    end = UI_JS.find(";", start)
+    raw = UI_JS[start:end].strip()
+    return int(raw)
+
+
+def test_upward_intent_window_is_widened_for_reading_pauses():
+    """The intent window must be >=2000ms so a brief reading pause during a
+    streaming response does not drop upward-scroll intent and re-pin the view
+    (#3250). The pre-fix 450ms value was too short for a real read-pause.
+    """
+    window = _intent_window_ms()
+    assert window >= 2000, (
+        f"MESSAGE_UPWARD_INTENT_MS is {window}ms; #3250 requires >=2000ms so a "
+        "reading pause during streaming does not expire upward-scroll intent "
+        "and snap the user back to the bottom."
+    )
+
+
+def test_intent_helper_compares_against_the_window_constant():
+    """_recentMessageUpwardIntent() must gate on the window constant, so the
+    widened value actually takes effect (guards against the helper being
+    rewritten with a hardcoded duration).
+    """
+    marker = "function _recentMessageUpwardIntent()"
+    idx = UI_JS.find(marker)
+    assert idx != -1, "_recentMessageUpwardIntent() not found in ui.js"
+    body = UI_JS[idx:UI_JS.find("}", idx) + 1]
+    assert "MESSAGE_UPWARD_INTENT_MS" in body, (
+        "_recentMessageUpwardIntent() must compare against MESSAGE_UPWARD_INTENT_MS "
+        "rather than a hardcoded duration (#3250)."
+    )
+    assert "_lastMessageUpwardIntentMs" in body, (
+        "_recentMessageUpwardIntent() must measure elapsed time since the last "
+        "recorded upward intent timestamp (#3250)."
+    )
+
+
+def test_downward_repin_is_independent_of_the_intent_window():
+    """Widening the intent window must not weaken downward re-pin: the movedUp
+    flag still requires an actual upward scrollTop delta (`top < _lastScrollTop
+    - 2`), so downward motion re-pins regardless of how long the intent window
+    is. This is what keeps the #3250 tuning safe.
+    """
+    anchor = "el.addEventListener('scroll'"
+    start = UI_JS.index(anchor)
+    raf_start = UI_JS.index("requestAnimationFrame", start)
+    brace = UI_JS.index("{", raf_start)
+    depth = 0
+    block = ""
+    for i in range(brace, len(UI_JS)):
+        ch = UI_JS[i]
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                block = UI_JS[brace:i + 1]
+                break
+    assert block, "scroll listener rAF callback not found"
+    moved_idx = block.index("const movedUp=")
+    moved_expr = block[moved_idx:block.find(";", moved_idx)]
+    assert "_lastScrollTop-2" in moved_expr or "_lastScrollTop -" in moved_expr, (
+        "movedUp must still require an explicit upward scrollTop delta so "
+        "downward motion re-pins independently of the intent window (#3250)."
+    )