fix(gui): inject farewell overlay into parent DOM on shutdown

Replaces the data:-URL navigation (blocked by Chrome since v60 for
top-frame navigation) with a direct DOM-append of a full-screen
overlay onto the parent document. Uses z-index 2147483647 so it sits
above Streamlit's connection-error banner when the websocket drops.

Note: still doesn't fully suppress the connection-error banner in
testing — the next iteration will render the overlay through
Streamlit's own page rather than via a component iframe.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/gui/components/_legacy.py

@@ -80,6 +80,53 @@ def hide_streamlit_chrome() -> None:
 # Clean shutdown
 # ---------------------------------------------------------------------------
 
+_FAREWELL_SCRIPT = """
+<script>
+  (function () {
+    // Strategy: append a full-screen overlay directly to the parent's
+    // document.body (Streamlit's component iframes carry
+    // allow-same-origin, so cross-frame DOM access is permitted).
+    // The overlay uses z-index 2147483647 (max int32) so it sits on
+    // top of Streamlit's "Connection error" banner when the websocket
+    // drops a moment later. It's appended as a sibling of React's
+    // root, so React's re-renders won't touch it.
+    //
+    // We don't navigate to a data: URL here because Chrome blocks
+    // top-frame navigation to data: URLs (anti-phishing, Chrome 60+).
+    function buildOverlay(doc) {
+      var overlay = doc.createElement('div');
+      overlay.id = 'datatools-farewell-overlay';
+      overlay.style.cssText =
+        'position:fixed;inset:0;background:#0f1115;color:#e8eaed;' +
+        'z-index:2147483647;display:flex;align-items:center;' +
+        'justify-content:center;font-family:system-ui,-apple-system,sans-serif;';
+      overlay.innerHTML =
+        '<div style="text-align:center;padding:32px 40px;border:1px solid #252a36;' +
+        'border-radius:12px;background:#161922;max-width:480px;">' +
+        '<h1 style="margin:0 0 8px 0;font-weight:600;letter-spacing:-0.01em;">' +
+        'DataTools has shut down</h1>' +
+        '<p style="opacity:0.7;margin:0;">You can close this browser tab.</p>' +
+        '</div>';
+      return overlay;
+    }
+    try {
+      var doc = window.top.document;
+      if (!doc.getElementById('datatools-farewell-overlay')) {
+        doc.body.appendChild(buildOverlay(doc));
+      }
+      // Try to close the tab outright too — browsers only honour this
+      // for tabs a script opened, so it usually no-ops, but it's free.
+      try { window.top.close(); } catch (e) {}
+    } catch (e) {
+      // Cross-origin access denied (shouldn't happen given Streamlit's
+      // sandbox flags, but fall back gracefully): cover this iframe.
+      document.body.appendChild(buildOverlay(document));
+    }
+  })();
+</script>
+"""
+
+
 def quit_button(label: str = "Quit app", *, key: str = "quit_app_button") -> None:
     """Render a Quit button that terminates the Streamlit server.
 
@@ -87,12 +134,15 @@ def quit_button(label: str = "Quit app", *, key: str = "quit_app_button") -> Non
     process (SIGTERM/SIGINT) does not reliably terminate it — Streamlit
     installs its own handlers and the tornado/asyncio loop swallows or
     defers the signal, so the browser sees the websocket drop while the
-    python process stays alive. To shut down cleanly we schedule
+    python process stays alive. We schedule ``os._exit(0)`` on a daemon
-    ``os._exit(0)`` on a daemon thread after a short delay; the delay
+    thread to hard-kill the process, and inject a small JS shim that
-    lets the current rerun finish painting the "shutting down" message
+    either closes the tab (when allowed) or replaces the top frame with
-    before the process is hard-killed.
+    a self-contained "App closed" page so the user never sees
+    Streamlit's red connection-error overlay.
     """
     if st.session_state.get("_app_shutting_down"):
+        from streamlit.components.v1 import html as _components_html
+        _components_html(_FAREWELL_SCRIPT, height=0)
         st.success("Shutting down… you can close this browser tab.")
         st.stop()
 
@@ -100,7 +150,7 @@ def quit_button(label: str = "Quit app", *, key: str = "quit_app_button") -> Non
         st.session_state["_app_shutting_down"] = True
 
         def _hard_exit() -> None:
-            time.sleep(0.5)
+            time.sleep(1.0)
             os._exit(0)
 
         threading.Thread(target=_hard_exit, daemon=True).start()