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, 14 insertions, 31 deletions

diff --git a/extensions/chrome-profile-bridge/browser-extension/service_worker.js b/extensions/chrome-profile-bridge/browser-extension/service_worker.js
index 0ceabda..d47b385 100644
--- a/extensions/chrome-profile-bridge/browser-extension/service_worker.js
+++ b/extensions/chrome-profile-bridge/browser-extension/service_worker.js
@@ -226,26 +226,12 @@ function isPiChromeOwnedTarget(tabId, sessionKey) {
   return false;
 }
 
-// The window agents share. Every session has its own tab group and a group cannot span windows,
-// but sessions do not need their own windows: new tabs go to the window that already holds any
-// "Pi Session:" group, so agents stay together instead of opening one window each. Returns
-// undefined when no such window exists (the first session then creates one).
-async function sharedPiWindowId() {
-  if (!chrome.tabGroups || typeof chrome.windows?.get !== "function") return undefined;
-  const groups = await chrome.tabGroups.query({}).catch(() => []);
-  for (const group of groups) {
-    if (!/^pi session:/i.test((group.title || "").trim()) || typeof group.windowId !== "number") continue;
-    const win = await chrome.windows.get(group.windowId).catch(() => null);
-    if (win && (win.type === undefined || win.type === "normal")) return win.id ?? group.windowId;
-  }
-  return undefined;
-}
-
 // Create a fresh automation target for `sessionKey`. If this session already has a tab group,
 // create the tab inside that group's window so one Pi session keeps one Chrome tab group (Chrome
-// groups cannot span windows). If it has none, use the shared Pi window; only when there is no
-// Pi window yet, create one; fall back to a tab. When the tab is created in a pre-existing window,
-// leave windowId unset so cleanup only closes our tab, never that whole window.
+// groups cannot span windows). If it has none, give the session its own window: a window shows
+// only one tab, so sessions sharing a window would hide each other's tabs, and a hidden tab gets no
+// real input and blank screenshots (measured 2026-11-08). Fall back to a tab. When the tab is
+// created in a pre-existing window, leave windowId unset so cleanup only closes our tab.
 async function createAutomationTarget(sessionKey, groupTitle) {
   const existingGroup = groupTitle ? await findGroupRecordByTitle(groupTitle) : null;
   if (existingGroup && typeof existingGroup.windowId === "number") {
@@ -254,14 +240,6 @@ async function createAutomationTarget(sessionKey, groupTitle) {
     await persistAutomationTargets();
     return tab;
   }
-  const sharedWindowId = await sharedPiWindowId();
-  if (sharedWindowId !== undefined) {
-    // Someone else's window: leave windowId unset so cleanup only ever closes our tab.
-    const tab = await chrome.tabs.create({ url: "about:blank", active: false, windowId: sharedWindowId });
-    automationTargets.set(sessionKey, { windowId: undefined, tabId: typeof tab.id === "number" ? tab.id : undefined });
-    await persistAutomationTargets();
-    return tab;
-  }
   if (chrome.windows && typeof chrome.windows.create === "function") {
     try {
       const win = await chrome.windows.create({ url: "about:blank", focused: false });
@@ -1911,15 +1889,20 @@ async function dispatch(action, params) {
       const existingGroup = await findGroupRecordByTitle(groupTitle);
       const createParams = { url: params.url || "about:blank", active: foregroundRequested(params) };
       if (existingGroup && typeof existingGroup.windowId === "number") createParams.windowId = existingGroup.windowId;
-      else {
-        // No group of this session yet: use the shared Pi window, never the user's focused one.
-        const sharedWindowId = await sharedPiWindowId();
-        if (sharedWindowId !== undefined) createParams.windowId = sharedWindowId;
+      // No group of this session yet: use the window of its automation tab if it has one, else open
+      // the tab in a window of its own. Never the user's focused window, where Chrome would put it.
+      let ownWindow = false;
+      if (createParams.windowId === undefined) {
+        const home = await resolveOwnedAutomationTarget(sessionKeyOf(params)).catch(() => null);
+        if (home && typeof home.windowId === "number") createParams.windowId = home.windowId;
+        else ownWindow = typeof chrome.windows?.create === "function";
       }
       // Register the creation as in flight until the tab is tracked, so early capture (which only
       // runs in tracked tabs) waits for it instead of skipping the new tab's first page load.
       const creation = (async () => {
-        const created = await chrome.tabs.create(createParams);
+        const created = ownWindow
+          ? (await chrome.windows.create({ url: createParams.url, focused: false })).tabs[0]
+          : await chrome.tabs.create(createParams);
         await trackSessionTab(sessionKeyOf(params), created.id, true);
         return created;
       })();