tabs: one window for every agent; inactive tabs take input; background off by defaultHEADmaster

Measured in Brave on 2026-11-14: an inactive tab takes trusted CDP clicks,
key presses, text and mouse moves, and Page.captureScreenshot renders it
live. Only mouseWheel hangs, waiting for a frame. The upstream guard that
refused input to every hidden page was wrong for inactive tabs, and it is
what made a window per session look necessary.

- New sessions' tabs open inactive in the user's last focused window, in
  the session's tab group; a window is created only when none is open.
- New groups start collapsed, unless their tab is the one showing, and
  expand when a tab in them is brought forward. Existing groups keep the
  user's setting.
- Input is refused up front only in a minimized window (untested there).
- chrome_scroll scrolls hidden pages by script instead of the wheel.
- Focus emulation on attach, so blur-sensitive menus stay open.
- Background mode is off by default. Calls still default to background;
  background:false or chrome_tab activate brings a tab forward.
- Agent guidance, tool descriptions and docs say so.

1 files changed, 10 insertions, 4 deletions

diff --git a/test-suite/unit/background-policy.test.mjs b/test-suite/unit/background-policy.test.mjs
index ea08be0..98eaa7c 100644
--- a/test-suite/unit/background-policy.test.mjs
+++ b/test-suite/unit/background-policy.test.mjs
@@ -71,7 +71,7 @@ function piHarness({ session = "alpha", send } = {}) {
 }
 
 const flagCases = [
-  [{}, false], [{ background: false }, false], [{ background: true }, true],
+  [{}, true], [{ background: false }, false], [{ background: true }, true],
   [{ foreground: true }, false], [{ foreground: false }, true],
   [{ background: false, foreground: false }, false], [{ background: true, foreground: true }, true],
 ];
@@ -96,6 +96,7 @@ test("session background on overrides all per-call/legacy foreground flags; off
 
 test("every registered page tool, tab.new, and chrome_launch(url) use the central policy", async () => {
   const h = piHarness();
+  await h.background("on");
   const cases = [
     ["chrome_launch", { url: "https://example.test" }, "tab.new"],
     ["chrome_tab", { action: "new", group: false, groupTitle: "other" }, "tab.new"],
@@ -143,7 +144,11 @@ test("typing options and verification pass through real tool registrations witho
 
 test("activation is rejected before dispatch; existing on/off/toggle/status commands suffice", async () => {
   const h = piHarness();
-  await assert.rejects(h.tool("chrome_tab", { action: "activate", targetId: "2", background: false }), /background mode.*\/chrome background off/);
+  await h.tool("chrome_tab", { action: "activate", targetId: "2" });
+  assert.equal(h.calls.at(-1).params.foreground, true, "activation is allowed by default");
+  h.calls.length = 0;
+  await h.background("on");
+  await assert.rejects(h.tool("chrome_tab", { action: "activate", targetId: "2", background: false }), /background mode on/);
   assert.equal(h.calls.length, 0);
   await h.background("off");
   await h.tool("chrome_tab", { action: "activate", targetId: "2" });
@@ -199,9 +204,9 @@ test("shared-owner sessions retain independent policies; per-call background sti
   const sharedCalls = [];
   const send = async (action, params) => { sharedCalls.push({ action, params }); return {}; };
   const a = piHarness({ session: "A", send }), b = piHarness({ session: "B", send });
-  await b.background("off");
+  await a.background("on");
   await a.send("tab.new", { background: false });
-  await b.send("tab.new", {});
+  await b.send("tab.new", { background: false });
   assert.equal(sharedCalls[0].params.sessionKey, "session:A");
   assert.equal(sharedCalls[0].params.foreground, false);
   assert.equal(sharedCalls[0].action, "tab.new.background");
@@ -314,6 +319,7 @@ function assertNoFocus(h) {
 test("Pi-to-worker background aliases preserve grouping, screenshot output, and ownership-aware cleanup", async () => {
   const h = workerHarness();
   const pi = piHarness({ send: (action, params) => h.w.dispatch(action, params) });
+  await pi.background("on");
   const opened = await pi.tool("chrome_tab", { action: "new", group: false, groupTitle: "wrong", background: false });
   assert.equal(opened.details.result.group.title, "Pi Session: alpha");
   const launched = await pi.tool("chrome_launch", { url: "https://fixture.test" });