tabs: back to one window per session; keep new tabs out of the user's window

A window shows only one tab, so sessions sharing one hid each other's tabs: no
real input and blank screenshots for every session but the one whose tab was
active (castle-pirc-web, 2026-11-08). Time-sharing the active tab would flip tabs
under the user while they sign in for an agent, so revert 66fbd30: a session
gets its own unfocused window. Kept: chrome_tab new for a session with no window
opens one for it instead of landing in the user's focused window. The primer
and chrome_tab description now say why to reuse a tab.

1 files changed, 10 insertions, 2 deletions

diff --git a/test-suite/unit/background-policy.test.mjs b/test-suite/unit/background-policy.test.mjs
index efbca78..ea08be0 100644
--- a/test-suite/unit/background-policy.test.mjs
+++ b/test-suite/unit/background-policy.test.mjs
@@ -297,6 +297,14 @@ function workerHarness({ withWindows = true } = {}) {
   };
   return h;
 }
+// A new Pi tab must not take over the user's window: it is either inactive, or the only tab of a
+// new, unfocused window of the session's own (so it is visible to the agent without stealing focus).
+function assertNewTabStaysOutOfTheWay(h, tab) {
+  if (tab.active === false) return;
+  assert.notEqual(tab.windowId, 1, "an active new tab must not be in the user's window");
+  assert.ok(h.calls.some((c) => c.action === "windows.create" && c.params.focused === false), "its window was created unfocused");
+}
+
 function assertNoFocus(h) {
   assert.equal(h.focusedWindow(), 1);
   assert.equal(h.tabs.get(1).active, true);
@@ -325,7 +333,7 @@ test("worker advertises support, keeps new tabs inactive, and rejects activation
   assert.equal((await h.w.dispatch("tab.version", {})).capabilities.hardBackground, true);
   for (const flags of [{}, { foreground: false }, { background: true, foreground: true }]) {
     const result = await h.w.dispatch("tab.new", { ...flags, sessionKey: "alpha", groupTitle: "Pi Session: alpha" });
-    assert.equal(result.tab.active, false);
+    assertNewTabStaysOutOfTheWay(h, result.tab);
     await assert.rejects(h.w.dispatch("tab.activate", { ...flags, targetId: "missing" }), /background mode/);
   }
   assertNoFocus(h);
@@ -339,7 +347,7 @@ test("worker advertises support, keeps new tabs inactive, and rejects activation
 test("background-only worker aliases force safe behavior even with foreground flags", async () => {
   const h = workerHarness();
   const opened = await h.w.dispatch("tab.new.background", { foreground: true, background: false });
-  assert.equal(opened.tab.active, false);
+  assertNewTabStaysOutOfTheWay(h, opened.tab);
   const shot = await h.w.dispatch("page.screenshot.background", { targetId: "2", foreground: true, background: false });
   assert.equal(shot.method, "cdp");
   assertNoFocus(h);