Kill the GraphRAG flash: warm the view up, and never paint it white

Two causes, both dealt with.

A fresh QWebEngineView paints white, and _ensure_web put it on screen empty —
so on a dark theme that white rectangle sat there for the whole first scan.
It is now blanked to the app's own background colour the moment it is created,
before it is ever shown.

And the work itself moved off the click. MainWindow warms GraphRAG up 3s after
the window appears — building the browser view (~140ms) and the first graph
(~485ms) while nothing is waiting on them. Clicking GraphRAG then costs 78ms
with zero scans and zero setHtml calls, where before it was ~625ms of empty
view.

This spends the memory the lazy construction was saving, a few seconds after
startup rather than never — which is the trade you asked for. Startup itself is
untouched: the checker asserts neither the view nor the graph exists at the
moment the window opens.

check_graphrag_rescan now covers the warm-up too, and fails if it stops running.

24/24 checkers pass; check_combo_popup took a teardown segfault in the suite and
passed 3/3 standalone.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tools/check_graphrag_rescan.py

@@ -50,24 +50,39 @@ def main() -> int:
     win.show()
     app.processEvents()
     st, w = win.structure, win.workspace
-    fails = []
+    fails: list[str] = []
 
-    # first visit builds the view and scans once
-    win._goto(win._ROW_WORKSPACE, w._graphrag_tab_idx)
+    # startup itself must not build any of it
+    if st.web is not None or st._graph is not None:
+        fails.append("dung san do thi ngay luc khoi dong — startup phai nhe")
+
+    # the idle warm-up builds the browser view and the first graph off the
+    # click path (MainWindow fires this on a 3s timer; call it directly here)
+    win._prewarm_graph()
     deadline = time.monotonic() + 30
     while st._graph is None and time.monotonic() < deadline:
         app.processEvents()
         time.sleep(0.02)
     if st._graph is None:
-        print("FAIL  lan dau khong dung duoc do thi")
+        print("FAIL  lam nong khong dung duoc do thi")
         sys.stdout.flush()
         os._exit(1)
+    print(f"sau khi lam nong: web={'co' if st.web else 'chua'} do thi={'co' if st._graph else 'chua'}")
 
     calls = {"scan": 0, "html": 0}
     real_scan, real_html = st._scan, st._render_d3
     st._scan = lambda: (calls.__setitem__("scan", calls["scan"] + 1), real_scan())[1]
     st._render_d3 = lambda: (calls.__setitem__("html", calls["html"] + 1), real_html())[1]
 
+    # ...so the click itself does nothing but show it — this is the flash
+    t0 = time.monotonic()
+    win._goto(win._ROW_WORKSPACE, w._graphrag_tab_idx)
+    app.processEvents()
+    print(f"bam vao GraphRAG: {(time.monotonic() - t0) * 1000:.0f}ms "
+          f"_scan={calls['scan']} setHtml={calls['html']}")
+    if calls["scan"] or calls["html"]:
+        fails.append("bam vao GraphRAG van phai quet/nap lai trang — con chop trang")
+
     # re-entering an unchanged graph must not rebuild anything
     for _ in range(3):
         win._goto(win._ROW_WORKSPACE, w._project_tab_idx)