diff --git a/electron/main.js b/electron/main.js
index f42d3d08..16495c53 100644
--- a/electron/main.js
+++ b/electron/main.js
@@ -1872,6 +1872,7 @@ function sendToRenderer(channel, ...args) {
 // Extracted to electron/updateErrorMessage.js so the mapping is unit-testable; see node --test there.
 const { friendlyUpdateError } = require('./updateErrorMessage');
 const { experimentalChannelDecision } = require('./experimentalChannelDecision');
+const { popupRoute } = require('./popupRoute');
 const { diagnoseSilentUpdateCheck } = require('./updateCheckDiagnosis');
 
 // Squirrel's built-in updater reports only via events; when AV or a proxy kills its request
@@ -2943,7 +2944,9 @@ app.on('web-contents-created', (_event, contents) => {
   }
 
   contents.setWindowOpenHandler(({ url, disposition }) => {
-    if (disposition === 'foreground-tab' || disposition === 'background-tab') {
+    // A popup opened by a browser card must become a card too, or it lands in a native window with
+    // no browser_id and the agent driving that card goes blind mid-auth (ENG-279).
+    if (popupRoute(contents.getType(), disposition) === 'card') {
       if (mainWindow && !mainWindow.isDestroyed()) {
         mainWindow.webContents.send('webview-new-window', url, contents.id, disposition);
       }
diff --git a/electron/popupRoute.js b/electron/popupRoute.js
new file mode 100644
index 00000000..317dfd4b
--- /dev/null
+++ b/electron/popupRoute.js
@@ -0,0 +1,29 @@
+// Where should a window.open from inside the app end up? (ENG-279)
+//
+// Two very different callers share one handler:
+//
+//   the MAIN window opening a provider sign-in (Anthropic's OAuth popup). That wants a real
+//   native child window and always has; it is app-level auth, nothing to do with browsing.
+//
+//   a BROWSER CARD's guest page opening "Sign in with Google", an SSO handoff, a consent or
+//   payment screen. That used to spawn a native window too, which sits outside the browser-card
+//   system and therefore has no browser_id. An agent driving that card simply cannot see it: the
+//   flow stops dead at the popup and the run looks stalled for no visible reason.
+//
+// So the opener decides. A tab disposition was already routed into a card; now anything opened by
+// a card goes to a card as well, which makes "a popup from a card that no agent can address"
+// unrepresentable rather than merely unlikely.
+
+/**
+ * @param {string} contentsType  webContents.getType(): 'webview' for a browser card's guest
+ * @param {string} disposition   Electron's window-open disposition
+ * @returns {'card'|'native'}
+ */
+function popupRoute(contentsType, disposition) {
+  if (disposition === 'foreground-tab' || disposition === 'background-tab') return 'card';
+  // Anything a browser card opens belongs to that card's world, whatever the disposition.
+  if (contentsType === 'webview') return 'card';
+  return 'native';
+}
+
+module.exports = { popupRoute };
diff --git a/electron/popupRoute.test.js b/electron/popupRoute.test.js
new file mode 100644
index 00000000..72418046
--- /dev/null
+++ b/electron/popupRoute.test.js
@@ -0,0 +1,32 @@
+// Run: node --test electron/popupRoute.test.js
+//
+// ENG-279. A "Sign in with Google" opened from inside a browser card used to become a native
+// window with no browser_id, so an agent driving that card could not see or touch it and the run
+// stalled with nothing on screen to explain why.
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { popupRoute } = require('./popupRoute');
+
+test('a popup opened BY a browser card becomes a card, so an agent can address it', () => {
+  for (const d of ['new-window', 'default', 'other', undefined]) {
+    assert.equal(popupRoute('webview', d), 'card', `disposition ${d} escaped the card system`);
+  }
+});
+
+test('provider sign-in from the main window keeps its native popup', () => {
+  for (const d of ['new-window', 'default', undefined]) {
+    assert.equal(popupRoute('window', d), 'native', `disposition ${d} hijacked provider auth`);
+  }
+});
+
+test('tab dispositions still route to a card from anywhere', () => {
+  for (const t of ['webview', 'window', 'browserView']) {
+    assert.equal(popupRoute(t, 'foreground-tab'), 'card');
+    assert.equal(popupRoute(t, 'background-tab'), 'card');
+  }
+});
+
+test('both routes are reachable, so the decision is not one-armed', () => {
+  const seen = new Set([popupRoute('webview', 'new-window'), popupRoute('window', 'new-window')]);
+  assert.equal(seen.size, 2, `only reached ${[...seen].join(', ')}`);
+});