fix: strip think tags when model emits leading whitespace before <think> (#327)

Remove ^ anchor from think/Gemma regexes in ui.js; trimStart() before startsWith checks in messages.js streaming path. Fixes MiniMax M2.7 and any model emitting leading newlines before <think>. 10 new tests, 768 total.

tests/test_sprint38.py

@@ -0,0 +1,111 @@
+"""
+Sprint 38 Tests: Think-tag stripping with leading whitespace (PR #327).
+
+Covers the static render path (ui.js regex logic, verified against the JS source)
+and the streaming render path (messages.js _streamDisplay logic).
+"""
+import pathlib
+import re
+
+REPO_ROOT = pathlib.Path(__file__).parent.parent
+UI_JS     = (REPO_ROOT / "static" / "ui.js").read_text()
+MSG_JS    = (REPO_ROOT / "static" / "messages.js").read_text()
+
+
+# ── ui.js: static render path ────────────────────────────────────────────────
+
+def test_think_regex_has_no_anchor():
+    """The <think> regex in ui.js must not use a ^ anchor so leading whitespace is allowed."""
+    # Find the thinkMatch line by locating the .match( call on that line
+    idx = UI_JS.find("const thinkMatch=content.match(")
+    assert idx >= 0, "thinkMatch line not found in ui.js"
+    line = UI_JS[idx:idx+100]
+    # The regex must NOT start with ^ right after the opening /
+    assert "/^<think>" not in line and "(/^" not in line, \
+        f"thinkMatch regex must not use ^ anchor — found: {line.strip()}"
+
+
+def test_gemma_regex_has_no_anchor():
+    """The Gemma channel-token regex in ui.js must not use a ^ anchor."""
+    match = re.search(r'const gemmaMatch=content\.match\((/[^/]+/)\)', UI_JS)
+    assert match, "gemmaMatch line not found in ui.js"
+    pattern = match.group(1)
+    assert not pattern.startswith('/^'), \
+        f"gemmaMatch regex must not use ^ anchor — got {pattern}"
+
+
+def test_think_content_removal_uses_replace_not_slice():
+    """After extracting thinkingText, content must use .replace() not .slice() to remove the tag."""
+    # Find the block that handles thinkMatch
+    idx = UI_JS.find("if(thinkMatch){")
+    assert idx >= 0, "thinkMatch handler block not found"
+    block = UI_JS[idx:idx+200]
+    assert "content.replace(" in block, \
+        "ui.js must use content.replace() to remove <think> block (not .slice())"
+    assert ".trimStart()" in block, \
+        "ui.js must call .trimStart() on content after removing the <think> block"
+
+
+def test_gemma_content_removal_uses_replace_not_slice():
+    """Gemma channel token removal must also use .replace() not .slice()."""
+    idx = UI_JS.find("if(gemmaMatch){")
+    assert idx >= 0, "gemmaMatch handler block not found"
+    block = UI_JS[idx:idx+200]
+    assert "content.replace(" in block, \
+        "ui.js must use content.replace() to remove Gemma channel block (not .slice())"
+    assert ".trimStart()" in block, \
+        "ui.js must call .trimStart() on content after removing the Gemma channel block"
+
+
+# ── messages.js: streaming render path ───────────────────────────────────────
+
+def test_stream_display_trims_before_startswith():
+    """_streamDisplay in messages.js must call .trimStart() before .startsWith() check."""
+    fn_idx = MSG_JS.find("function _streamDisplay()")
+    assert fn_idx >= 0, "_streamDisplay function not found in messages.js"
+    fn_end = MSG_JS.find("\n  }", fn_idx) + 4
+    fn_body = MSG_JS[fn_idx:fn_end]
+    assert "trimStart()" in fn_body, \
+        "_streamDisplay must call trimStart() to handle models that emit leading whitespace before <think>"
+
+
+def test_stream_display_uses_trimmed_for_startswith():
+    """_streamDisplay must check trimmed.startsWith(open), not raw.startsWith(open)."""
+    fn_idx = MSG_JS.find("function _streamDisplay()")
+    fn_end = MSG_JS.find("\n  }", fn_idx) + 4
+    fn_body = MSG_JS[fn_idx:fn_end]
+    assert "trimmed.startsWith(open)" in fn_body, \
+        "_streamDisplay must use trimmed.startsWith(open) not raw.startsWith(open)"
+
+
+def test_stream_display_partial_tag_uses_trimmed():
+    """The partial-tag guard in _streamDisplay must also use trimmed, not raw."""
+    fn_idx = MSG_JS.find("function _streamDisplay()")
+    fn_end = MSG_JS.find("\n  }", fn_idx) + 4
+    fn_body = MSG_JS[fn_idx:fn_end]
+    assert "open.startsWith(trimmed)" in fn_body, \
+        "Partial-tag guard must use open.startsWith(trimmed) not open.startsWith(raw)"
+
+
+def test_stream_display_trims_return_after_close():
+    """After stripping a completed think block, _streamDisplay must trim leading whitespace from the result."""
+    fn_idx = MSG_JS.find("function _streamDisplay()")
+    fn_end = MSG_JS.find("\n  }", fn_idx) + 4
+    fn_body = MSG_JS[fn_idx:fn_end]
+    # The return after finding close must strip whitespace from the result
+    assert ".replace(/^" in fn_body and "s+/,'')" in fn_body, \
+        "_streamDisplay must strip leading whitespace from content after the closing think tag"
+
+
+# ── Regression: existing anchored patterns must be gone ──────────────────────
+
+def test_no_anchored_think_regex_in_ui_js():
+    """The old anchored regex /^<think>/ must not exist in ui.js."""
+    assert "/^<think>" not in UI_JS, \
+        "Old anchored /^<think>/ regex still present in ui.js — fix not applied"
+
+
+def test_no_anchored_gemma_regex_in_ui_js():
+    """The old anchored Gemma regex must not exist in ui.js."""
+    assert "/^<|channel>" not in UI_JS, \
+        "Old anchored /^<|channel>/ regex still present in ui.js — fix not applied"