#5390: don't preserve dead empty live-turn shell across DOM wipe (blank assistant turn), rebased on master

tests/test_blank_live_turn_preserve_guard.py

@@ -0,0 +1,113 @@
+"""Regression: blank assistant turn (对话消失) — dead empty live-turn shell
+survives a session-updated swap re-render and hides the settled answer.
+
+Root cause (reproduced + fixed on an isolated debug instance, 2026-07-01)
+------------------------------------------------------------------------
+`renderMessages()` (static/ui.js) preserves the `#liveAssistantTurn` DOM node
+across the `inner.innerHTML=''` wipe so the smd parser's live reference is not
+detached mid-stream (#3877 flicker fix). The preserve guard originally fired
+whenever `INFLIGHT[sid]` existed:
+
+    let _preservedLiveTurn=null;
+    if(sid&&INFLIGHT[sid]){
+      const _lt=document.getElementById('liveAssistantTurn');
+      if(_lt&&(...sessionId matches...)){ _preservedLiveTurn=_lt; }
+    }
+
+When a turn's SSE dropped (S.activeStreamId cleared to null) but its
+`INFLIGHT[sid]` entry was NOT cleaned, the live turn was a DEAD empty shell —
+avatar + an empty worklog group ("Processed Ns", no body/tool rows). On the
+next `session-updated` self-heal swap (loadSession force + keepStaleUntilLoaded,
+common under repeated self-wake restarts), the guard re-attached that empty
+shell OVER the freshly-wiped transcript, pinning an avatar-only blank turn on
+top of the already-persisted answer. That is the reported "对话消失".
+
+Fix
+---
+Preserve the live turn ONLY when it is genuinely live: an active stream is
+still running (`S.activeStreamId`) — the #3877 case — OR it already holds real
+rendered content (`.msg-body`, `.tool-card-row`, or `.wl-reason`). A dead empty
+shell (no content, no active stream) is no longer preserved, so the swap wipe
+drops it and the settled transcript renders normally.
+"""
+import pathlib
+import re
+import shutil
+import subprocess
+import textwrap
+
+REPO = pathlib.Path(__file__).parent.parent
+
+
+def read(rel):
+    return (REPO / rel).read_text(encoding="utf-8")
+
+
+def _preserve_guard_src():
+    src = read("static/ui.js")
+    i = src.find("let _preservedLiveTurn=null;")
+    assert i >= 0, "_preservedLiveTurn guard not found"
+    # capture through the closing of the if-block (next 'const compressionState')
+    j = src.find("const compressionState", i)
+    assert j > i, "guard block end not found"
+    return src[i:j]
+
+
+class TestBlankLiveTurnPreserveGuard:
+    def test_guard_requires_real_content_or_active_stream(self):
+        guard = _preserve_guard_src()
+        # Must gate the preserve on real content OR an active stream — not merely
+        # on INFLIGHT existence.
+        assert "_hasRealLiveContent" in guard, (
+            "preserve guard must compute whether the live turn has real content"
+        )
+        assert ".msg-body" in guard and ".tool-card-row" in guard and ".wl-reason" in guard, (
+            "real-content check must look for a visible body / tool card / reason row"
+        )
+        assert "S.activeStreamId" in guard, (
+            "preserve guard must still preserve a genuinely-streaming turn (#3877)"
+        )
+        # The assignment must be inside the new conditional.
+        assert re.search(
+            r"if\(_hasRealLiveContent\s*\|\|\s*S\.activeStreamId\)\{\s*_preservedLiveTurn=_lt;",
+            guard,
+        ), "preserve assignment must be gated by (hasRealContent || activeStreamId)"
+
+    def test_runtime_rejects_dead_shell_preserves_live(self):
+        node = shutil.which("node")
+        if not node:
+            import pytest
+            pytest.skip("node not available")
+        script = textwrap.dedent(
+            """
+            const assert=require('assert');
+            // Mirror the guard's decision predicate exactly.
+            function guardWouldPreserve(lt, activeStreamId){
+              if(!lt) return false;
+              const hasReal=!!lt.querySelector('.msg-body, .tool-card-row, .wl-reason');
+              return hasReal || !!activeStreamId;
+            }
+            // Minimal DOM element stub with querySelector over a class set.
+            function el(classes){
+              const set=new Set(classes||[]);
+              return { querySelector(sel){
+                // sel is a comma list of .class tokens
+                return sel.split(',').map(s=>s.trim().replace(/^\\./,''))
+                  .some(c=>set.has(c)) ? {} : null;
+              }};
+            }
+            const deadShell = el([]);                 // empty worklog shell, no content
+            const withBody  = el(['msg-body']);
+            const withTool  = el(['tool-card-row']);
+            const withReason= el(['wl-reason']);
+            assert.strictEqual(guardWouldPreserve(deadShell, null), false, 'dead shell must NOT be preserved');
+            assert.strictEqual(guardWouldPreserve(deadShell, 'sid'), true, 'streaming empty shell preserved (#3877)');
+            assert.strictEqual(guardWouldPreserve(withBody, null), true, 'body content preserved');
+            assert.strictEqual(guardWouldPreserve(withTool, null), true, 'tool card preserved');
+            assert.strictEqual(guardWouldPreserve(withReason, null), true, 'reason row preserved');
+            console.log('OK');
+            """
+        )
+        out = subprocess.run([node, "-e", script], capture_output=True, text=True)
+        assert out.returncode == 0, f"node harness failed: {out.stderr}\n{out.stdout}"
+        assert "OK" in out.stdout