feat(windows): support multi-window tab workflows

docs/feature-checklist.md

@@ -265,6 +265,8 @@ Flagship feature. C++ services + `dao://dao-agent` WebUI + vendor runtime.
 |---|---------|--------------------|------|----------------------|
 | ☐ | Split container / panes / divider | `src/dao/.../split/` + `dao_colors.*`, `dao_corner_overlay_view.cc`, `contents_layout_manager.cc.patch`, `browser_view.cc.patch` | 🟡 | Split creates two panes (wired but off by default); horizontal and vertical dividers and inner pane corner cutouts match the shadowed content-frame edge in light/dark and incognito windows, including theme changes while split; outer corner shadows stay unchanged, single-pane content remains visible, bright pages do not lighten the opaque divider, and blue hover/drag feedback remains visible |
 | ☐ | Native tab-drag cleanup restores WebContents hit testing | `dao_native_util_mac.mm`, `split/dao_split_view.cc` | 🟡 | Cancel a tab drag over or outside the content area, then verify existing and newly opened tabs still receive mouse input in every window |
+| ☐ | New-window and existing-window tab transfer | `dao_cross_window_drag.*`, `dao_native_util_mac.*`, `dao_sidebar_ui.cc`, sidebar WebUI tests | `DaoCrossWindowDragBrowserTest.*`, `DaoTabDragMacBrowserTest.*` | Right-click a background tab and move that exact tab to a new window; the last-tab menu action is disabled. Drag one of several tabs onto the desktop and preserve its contents/history; drag the only tab and move its window. Escape must not create a window or remove folder membership. Successful tab-list and Split View drops create no extra window; closing/reordering the dragged tab never moves a different tab. Test all display arrangements and hidden/minimized windows, including a source display with a different logical height from the primary display, negative screen coordinates, and fractional drop positions. |
+| ☐ | Window-scoped folder persistence | `dao_folder_storage.*`, `dao_tab_identity.*`, `dao_folder_model.ts`, `dao_sidebar_ui.cc` | Folder storage and sidebar WebUI tests | Create different folders in two windows, save them alternately, restart with session restore, and verify each window retains its own folders and duplicate-URL tab identities. Opening a new empty window must not claim another window's folders. Move a tab across windows and verify unrelated folders remain intact; reject a foreign-window tab dropped directly onto a folder. Verify legacy file migration, imported-tab folders, and failed atomic writes preserve existing data. Incognito folder changes must remain in memory and leave the regular profile file untouched. |
 | ☐ | No crash on pane reparent double-detach (macOS) | `content/.../web_contents_view_mac.mm.patch` (null-guard) | 🟡 | Reparenting panes doesn't crash |
 | ☐ | No fatal paint-chunk-duplicate during reparent | `blink/.../paint/paint_controller.cc.patch` (fatal→`LOG(ERROR)`) | 🟡 | Split deactivation doesn't crash debug build |
 

docs/features.md

@@ -52,7 +52,13 @@ An Arc-inspired vertical sidebar replaces Chromium's top tab strip — the singl
   the tab count. Pin state is serialized through a shared sequenced writer and
   atomically replaces the previous profile file.
 - **dao_folder_item.ts** / **dao_folder_model.ts** — Folder grouping with
-  profile-path persistence (load/save round-trip). Every folder context menu
+  window-scoped persistence under the profile path. Stable tab identities
+  preserve folder membership across session restore and distinguish tabs with
+  identical URLs. Window snapshots merge in shared profile state and are written
+  atomically so one window cannot overwrite another window's folders. Legacy
+  folder files remain readable. Off-the-record windows keep folder snapshots in
+  isolated memory and never read or write the regular profile's folder file.
+  Every folder context menu
   exposes Unfolder, which removes the folder and releases its child tabs in
   place, and Delete Folder, which uses a Dao native system confirmation dialog
   before closing the folder's currently matched child tabs. The folder is
@@ -72,7 +78,20 @@ An Arc-inspired vertical sidebar replaces Chromium's top tab strip — the singl
 ### 1.3 Tab System Foundations
 - **DaoTabIdentity** (`dao_tab_identity.h`) — Stable cross-window tab IDs decoupled from `TabStripModel` indices, migrated across WebContents replacement and persisted in session extra data
 - **DaoTabCommands** (`dao_tab_commands.h`) — Tab action vocabulary (duplicate, pin, copy URL, close, etc.)
-- **DaoCrossWindowDrag** (`dao_cross_window_drag.{h,cc}`) — `dao-tab-drag:<session_id>:<tab_index>` pasteboard payload + parser, shared by `dao_tab_item.ts` and the macOS drop handler in `dao_native_util_mac.mm`
+- **DaoCrossWindowDrag** (`dao_cross_window_drag.{h,cc}`) — Shared parsing and
+  tab resolution for `dao-tab-drag:<session_id>:<tab_index>:<tab_id>` payloads;
+  legacy index-only identities remain readable. Stable IDs prevent moving a
+  different tab when the source window reorders during a drag. Existing-window
+  and Split View drops use the same transfer helper.
+- **Move tab to a new window** — The ordinary tab context menu uses Chromium's
+  native move command and localized label. Dragging a tab onto the desktop
+  creates a window before transferring its contents; dragging the only tab
+  moves the existing window. Desktop drop detection and window placement use
+  primary-display global coordinates, including on displays of different sizes.
+  Native drag completion controls tear-off, while
+  successful drops and Escape cancellation only clean up drag state. Cross-window
+  folder drops are rejected; ordinary tab-list drops never remove a destination
+  window's folder membership by interpreting a foreign tab index.
 - **Detach guards** — Prevents accidental reordering while dragging
 - Patches: `tab_strip_model.cc.patch`, `tab_helpers.cc.patch`
 

src/dao/browser/import/dao_chromium_migration_target.cc

@@ -21,6 +21,8 @@
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/resource_coordinator/lifecycle_unit_state.mojom.h"
 #include "chrome/browser/resource_coordinator/tab_lifecycle_unit_external.h"
+#include "chrome/browser/sessions/session_service.h"
+#include "chrome/browser/sessions/session_service_factory.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_finder.h"
 #include "chrome/browser/ui/browser_tabstrip.h"
@@ -37,8 +39,8 @@
 #include "components/password_manager/core/browser/password_store/password_store_interface.h"
 #include "components/password_manager/core/common/password_manager_pref_names.h"
 #include "components/prefs/pref_service.h"
+#include "components/sessions/content/session_tab_helper.h"
 #include "dao/browser/ui/views/dao_tab_identity.h"
-#include "dao/browser/ui/webui/dao_pinned_tab_storage.h"
 #include "dao/browser/ui/webui/dao_sidebar_ui.h"
 #include "extensions/browser/disable_reason.h"
 #include "extensions/browser/extension_registrar.h"
@@ -62,6 +64,24 @@ std::pair<std::string, std::u16string> PasswordKey(const PasswordEntry& entry) {
   return {entry.signon_realm, entry.username};
 }
 
+Browser* FindBrowserBySessionId(Profile* profile, SessionID session_id) {
+  ProfileBrowserCollection* collection =
+      ProfileBrowserCollection::GetForProfile(profile);
+  if (!collection) {
+    return nullptr;
+  }
+  Browser* result = nullptr;
+  collection->ForEach([&](BrowserWindowInterface* browser_window) {
+    Browser* browser = browser_window->GetBrowserForMigrationOnly();
+    if (browser && browser->session_id() == session_id) {
+      result = browser;
+      return false;
+    }
+    return true;
+  });
+  return result;
+}
+
 }  // namespace
 
 DaoChromiumMigrationTarget::DaoChromiumMigrationTarget(Profile* profile)
@@ -321,13 +341,15 @@ bool DaoChromiumMigrationTarget::AddDormantTab(const TabEntry& entry,
   if (!pending_id || *pending_id != folder_id) {
     return false;
   }
-  BrowserWindowInterface* browser_window =
-      chrome::FindLastActiveWithProfile(profile_);
-  if (!browser_window) {
+  Browser* browser =
+      pending_folder_window_id_
+          ? FindBrowserBySessionId(profile_, *pending_folder_window_id_)
+          : nullptr;
+  if (!browser) {
     return false;
   }
   content::WebContents* contents =
-      chrome::AddAndReturnTabAt(browser_window, entry.url, -1, false);
+      chrome::AddAndReturnTabAt(browser, entry.url, -1, false);
   if (!contents) {
     return false;
   }
@@ -355,18 +377,38 @@ bool DaoChromiumMigrationTarget::FinishImportedTabFolder(
   if (!pending_id || *pending_id != folder_id) {
     return false;
   }
-  bool persisted = false;
-  std::string json;
-  if (base::JSONWriter::WriteWithOptions(
-          folder_data_, base::JSONWriter::OPTIONS_PRETTY_PRINT, &json)) {
-    persisted = WritePinnedTabsFileAtomically(
-        profile_->GetPath().AppendASCII("dao_folders.json"), json);
-    if (persisted) {
-      DaoSidebarUIHandler::NotifyFolderDataChanged(profile_);
-    }
+  Browser* browser =
+      pending_folder_window_id_
+          ? FindBrowserBySessionId(profile_, *pending_folder_window_id_)
+          : nullptr;
+  if (!browser) {
+    return false;
   }
+  std::set<std::string> tab_ids;
+  TabStripModel* tab_strip = browser->tab_strip_model();
+  for (int index = 0; index < tab_strip->count(); ++index) {
+    content::WebContents* contents = tab_strip->GetWebContentsAt(index);
+    tab_ids.insert(GetSidebarTabId(contents));
+  }
+  const bool persisted = DaoSidebarUIHandler::PersistImportedFolder(
+      profile_, folder_snapshot_id_, std::move(tab_ids),
+      pending_folder_->Clone());
   if (persisted) {
+    SessionService* session_service =
+        SessionServiceFactory::GetForProfile(profile_);
+    for (int index = 0; index < tab_strip->count(); ++index) {
+      content::WebContents* contents = tab_strip->GetWebContentsAt(index);
+      SetSidebarFolderSnapshotId(contents, folder_snapshot_id_);
+      sessions::SessionTabHelper* session_tab_helper =
+          sessions::SessionTabHelper::FromWebContents(contents);
+      if (session_service && session_tab_helper) {
+        session_service->AddTabExtraData(
+            session_tab_helper->window_id(), session_tab_helper->session_id(),
+            kSidebarFolderSnapshotSessionKey, folder_snapshot_id_);
+      }
+    }
     pending_folder_ = nullptr;
+    pending_folder_window_id_.reset();
     pending_folder_tab_ids_.clear();
   }
   return persisted;
@@ -382,6 +424,7 @@ void DaoChromiumMigrationTarget::AbortImportedTabFolder(
     });
   }
   pending_folder_ = nullptr;
+  pending_folder_window_id_.reset();
 
   ProfileBrowserCollection* collection =
       ProfileBrowserCollection::GetForProfile(profile_);
@@ -552,20 +595,49 @@ DaoChromiumMigrationTarget::FindOrCreateBookmarkParent(
 }
 
 bool DaoChromiumMigrationTarget::LoadFolderData() {
-  std::string contents;
-  const base::FilePath path =
-      profile_->GetPath().AppendASCII("dao_folders.json");
-  if (base::PathExists(path)) {
-    if (!base::ReadFileToString(path, &contents)) {
-      return false;
+  pending_folder_window_id_.reset();
+  folder_snapshot_id_.clear();
+  folder_data_.clear();
+  folder_items_ = nullptr;
+  pending_folder_ = nullptr;
+  pending_folder_tab_ids_.clear();
+  std::vector<std::string> preferred_ids;
+  std::set<std::string> tab_ids;
+  BrowserWindowInterface* browser_window =
+      chrome::FindLastActiveWithProfile(profile_);
+  if (!browser_window) {
+    return false;
+  }
+  Browser* browser = browser_window->GetBrowserForMigrationOnly();
+  pending_folder_window_id_ = browser->session_id();
+  TabStripModel* tab_strip = browser->tab_strip_model();
+  std::set<std::string> seen_snapshot_ids;
+  for (int index = 0; index < tab_strip->count(); ++index) {
+    content::WebContents* tab = tab_strip->GetWebContentsAt(index);
+    tab_ids.insert(GetSidebarTabId(tab));
+    std::string snapshot_id = GetSidebarFolderSnapshotId(tab);
+    if (!snapshot_id.empty() && seen_snapshot_ids.insert(snapshot_id).second) {
+      preferred_ids.push_back(std::move(snapshot_id));
     }
+  }
+
+  std::string snapshot_json;
+  if (!DaoSidebarUIHandler::LoadFolderSnapshotForImport(
+          profile_, preferred_ids, tab_ids, &folder_snapshot_id_,
+          &snapshot_json)) {
+    return false;
+  }
+  if (!snapshot_json.empty()) {
     std::optional<base::DictValue> parsed =
-        base::JSONReader::ReadDict(contents, base::JSON_PARSE_RFC);
+        base::JSONReader::ReadDict(snapshot_json, base::JSON_PARSE_RFC);
     if (!parsed) {
       return false;
     }
     folder_data_ = std::move(*parsed);
   } else {
+    if (folder_snapshot_id_.empty()) {
+      folder_snapshot_id_ = base::Uuid::GenerateRandomV4().AsLowercaseString();
+    }
     folder_data_.Set("version", 1);
     folder_data_.Set("items", base::ListValue());
   }

src/dao/browser/import/dao_chromium_migration_target.h

@@ -20,6 +20,7 @@
 #include "base/task/cancelable_task_tracker.h"
 #include "base/values.h"
 #include "components/password_manager/core/browser/password_store/password_store_consumer.h"
+#include "components/sessions/core/session_id.h"
 #include "dao/browser/import/dao_migration_writer.h"
 
 class Profile;
@@ -112,9 +113,11 @@ class DaoChromiumMigrationTarget
   ItemWriteCallback pending_password_callback_;
   std::map<std::string, raw_ptr<const bookmarks::BookmarkNode>>
       bookmark_folders_;
+  std::string folder_snapshot_id_;
   base::DictValue folder_data_;
   raw_ptr<base::ListValue> folder_items_ = nullptr;
   raw_ptr<base::DictValue> pending_folder_ = nullptr;
+  std::optional<SessionID> pending_folder_window_id_;
   std::vector<std::string> pending_folder_tab_ids_;
   std::deque<ExtensionEntry> extension_queue_;
   scoped_refptr<extensions::WebstoreInstallWithPrompt> extension_installer_;

src/dao/browser/ui/dao_ui_sources.gni

@@ -215,6 +215,8 @@ dao_browser_ui_sources = [
   "//dao/browser/ui/webui/dao_home_ui.h",
   "//dao/browser/ui/webui/dao_import_ui.cc",
   "//dao/browser/ui/webui/dao_import_ui.h",
+  "//dao/browser/ui/webui/dao_folder_storage.cc",
+  "//dao/browser/ui/webui/dao_folder_storage.h",
   "//dao/browser/ui/webui/dao_pinned_tab_model.cc",
   "//dao/browser/ui/webui/dao_pinned_tab_model.h",
   "//dao/browser/ui/webui/dao_pinned_tab_storage.cc",
@@ -269,11 +271,13 @@ dao_browser_ui_test_sources = [
   "//dao/browser/automation/dao_page_tools_browsertest.cc",
   "//dao/browser/automation/dao_tab_tools_browsertest.cc",
   "//dao/browser/ui/views/dao_browser_browsertest.cc",
+  "//dao/browser/ui/webui/dao_folder_storage_unittest.cc",
   "//dao/browser/ui/webui/dao_pinned_tab_model_unittest.cc",
 ]
 
 dao_browser_ui_mac_test_sources = [
   "//dao/browser/mcp/dao_mcp_helper_browsertest.cc",
+  "//dao/browser/ui/views/dao_tab_drag_mac_browsertest.mm",
   "//dao/browser/ui/views/dao_test_init_mac.mm",
 ]