[FocusMode] Introduce TabStripPlacementCoordinator (#36255)

Adds a TabStripPlacementCoordinator that owns the parenting of the
horizontal tab strip region view and selects an appropriate parent based
on browser state (vertical tabs, focus mode, default). Callers register
parents per PlacementKind and trigger reassignment via UpdatePlacement.

Migrates BraveVerticalTabStripRegionView off its ad-hoc reparenting
onto the coordinator.

The kTopContainer placement is wired up but left unused; Focus Mode
will register a parent for it in a follow-up.

browser/ui/BUILD.gn

@@ -731,7 +731,7 @@ source_set("ui") {
       "//brave/browser/ui/side_panel",
       "//brave/browser/ui/sidebar/buildflags",
       "//brave/browser/ui/views:layout_provider",
-      "//brave/browser/ui/views/frame",
+      "//brave/browser/ui/views/frame:frame_impl",
       "//brave/browser/ui/views/location_bar",
       "//brave/browser/ui/views/page_action",
       "//brave/browser/ui/views/tabs/accent_color",

browser/ui/config.gni

@@ -51,6 +51,7 @@ if (toolkit_views) {
     "//brave/browser/ui/split_view",
     "//brave/browser/ui/views:layout_provider",
     "//brave/browser/ui/views/frame",
+    "//brave/browser/ui/views/frame:frame_impl",
     "//brave/browser/ui/views/frame/split_view",
     "//brave/browser/ui/views/frame/vertical_tabs",
     "//brave/browser/ui/views/location_bar",

browser/ui/views/frame/BUILD.gn

@@ -5,30 +5,45 @@
 
 source_set("frame") {
   sources = [
-    "brave_contents_view_util.cc",
     "brave_contents_view_util.h",
     "layout/brave_browser_view_layout_delegate_impl.cc",
     "layout/brave_browser_view_layout_delegate_impl.h",
     "layout/brave_browser_view_tabbed_layout_impl.cc",
     "layout/brave_browser_view_tabbed_layout_impl.h",
+    "tab_strip_placement_coordinator.h",
+  ]
+
+  public_deps = [
+    "//base",
+    "//ui/views",
   ]
 
   deps = [
-    "//base",
     "//brave/browser/ui:brave_tab_prefs",
-    "//brave/browser/ui/views/frame/vertical_tabs",
     "//brave/browser/ui/views/sidebar",
+    "//chrome/browser/ui/exclusive_access",
+    "//chrome/browser/ui/views/infobars",
+    "//ui/compositor",
+  ]
+}
+
+source_set("frame_impl") {
+  sources = [
+    "brave_contents_view_util.cc",
+    "tab_strip_placement_coordinator.cc",
+  ]
+
+  deps = [
+    ":frame",
+    "//brave/browser/ui/focus_mode",
+    "//brave/browser/ui/views/frame/vertical_tabs",
     "//chrome/browser/profiles:profile",
     "//chrome/browser/ui/browser_window",
-    "//chrome/browser/ui/exclusive_access",
     "//chrome/browser/ui/tabs:tab_strip",
-    "//chrome/browser/ui/views/infobars",
     "//chrome/common:constants",
     "//components/prefs",
     "//components/split_tabs",
     "//components/tabs:public",
-    "//ui/compositor",
-    "//ui/views",
   ]
 }
 
@@ -54,11 +69,14 @@ source_set("browser_tests") {
   testonly = true
   defines = [ "HAS_OUT_OF_PROC_TEST_RUNNER" ]
 
-  sources =
-      [ "//brave/browser/ui/views/frame/brave_browser_view_browsertest.cc" ]
+  sources = [
+    "//brave/browser/ui/views/frame/brave_browser_view_browsertest.cc",
+    "//brave/browser/ui/views/frame/tab_strip_placement_coordinator_browsertest.cc",
+  ]
 
   deps = [
     ":frame",
+    ":frame_impl",
     "//base",
     "//brave/browser/ui/views/frame/vertical_tabs",
     "//brave/browser/ui/views/sidebar",

browser/ui/views/frame/brave_browser_view.cc

@@ -36,6 +36,7 @@
 #include "brave/browser/ui/views/frame/brave_contents_view_util.h"
 #include "brave/browser/ui/views/frame/split_view/brave_contents_container_view.h"
 #include "brave/browser/ui/views/frame/split_view/brave_multi_contents_view.h"
+#include "brave/browser/ui/views/frame/tab_strip_placement_coordinator.h"
 #include "brave/browser/ui/views/frame/vertical_tabs/vertical_tab_strip_region_view.h"
 #include "brave/browser/ui/views/frame/vertical_tabs/vertical_tab_strip_widget_delegate_view.h"
 #include "brave/browser/ui/views/location_bar/brave_location_bar_view.h"
@@ -306,6 +307,10 @@ bool BraveBrowserView::ShouldUseBraveWebViewRoundedCornersForContents(
 }
 
 BraveBrowserView::BraveBrowserView(Browser* browser) : BrowserView(browser) {
+  tab_strip_placement_ = std::make_unique<TabStripPlacementCoordinator>(
+      base::PassKey<BraveBrowserView>(), browser,
+      horizontal_tab_strip_region_view_);
+
   // Need this background view always as we have contents margin/rounded corners
   // when split view is active regardless of rounded corners feature.
   contents_background_view_ =

browser/ui/views/frame/brave_browser_view.h

@@ -70,6 +70,7 @@ class BraveMultiContentsView;
 class ContentsLayoutManager;
 class SidebarContainerView;
 class SidePanelEntry;
+class TabStripPlacementCoordinator;
 class VerticalTabStripWidgetDelegateView;
 class ViewShadow;
 
@@ -168,6 +169,10 @@ class BraveBrowserView : public BrowserView,
     return sidebar_container_view_;
   }
 
+  TabStripPlacementCoordinator* tab_strip_placement_coordinator() {
+    return tab_strip_placement_.get();
+  }
+
 #if BUILDFLAG(ENABLE_EXTENSIONS)
   // Returns the PWA Shields toolbar button, if it exists. Note that this
   // returns valid pointer only when it's web app browser.
@@ -271,6 +276,7 @@ class BraveBrowserView : public BrowserView,
     return top_container_separator_;
   }
 
+  std::unique_ptr<TabStripPlacementCoordinator> tab_strip_placement_;
   std::unique_ptr<views::Widget> vertical_tab_strip_widget_;
 
   bool closing_confirm_dialog_activated_ = false;

browser/ui/views/frame/tab_strip_placement_coordinator.cc

@@ -0,0 +1,84 @@
+/* Copyright (c) 2026 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#include "brave/browser/ui/views/frame/tab_strip_placement_coordinator.h"
+
+#include "base/check_deref.h"
+#include "brave/browser/ui/focus_mode/focus_mode_utils.h"
+#include "brave/browser/ui/views/tabs/vertical_tab_utils.h"
+#include "ui/views/view.h"
+
+TabStripPlacementCoordinator::TabStripPlacementCoordinator(
+    base::PassKey<BraveBrowserView>,
+    BrowserWindowInterface* browser_window_interface,
+    views::View* tab_strip_region_view)
+    : browser_window_interface_(CHECK_DEREF(browser_window_interface)),
+      tab_strip_region_view_(CHECK_DEREF(tab_strip_region_view)) {
+  auto* original_parent = tab_strip_region_view_->parent();
+  CHECK(original_parent);
+
+  auto original_index =
+      original_parent->GetIndexOf(base::to_address(tab_strip_region_view_));
+
+  SetPlacement(PlacementKind::kDefault, original_parent, original_index);
+}
+
+TabStripPlacementCoordinator::~TabStripPlacementCoordinator() {
+  // On destruction, restore the tabstrip to its original placement so that
+  // browser view teardown runs as expected. Clear all placements other than the
+  // default placement and trigger an update.
+  for (auto& [kind, placement] : placements_) {
+    if (kind != PlacementKind::kDefault) {
+      placement = {};
+    }
+  }
+  UpdatePlacement();
+}
+
+void TabStripPlacementCoordinator::SetPlacement(PlacementKind kind,
+                                                views::View* parent,
+                                                std::optional<size_t> index) {
+  placements_[kind] = {.parent = parent, .index = index};
+}
+
+void TabStripPlacementCoordinator::ClearPlacement(PlacementKind kind) {
+  placements_[kind] = {};
+}
+
+void TabStripPlacementCoordinator::UpdatePlacement() {
+  auto* browser = base::to_address(browser_window_interface_);
+
+  auto get_placement = [&]() -> const Placement& {
+    if (tabs::utils::ShouldShowBraveVerticalTabs(browser)) {
+      auto& placement = placements_[PlacementKind::kVerticalTabStrip];
+      if (placement.parent) {
+        return placement;
+      }
+    }
+    if (IsFocusModeEnabled(browser)) {
+      auto& placement = placements_[PlacementKind::kTopContainer];
+      if (placement.parent) {
+        return placement;
+      }
+    }
+    return placements_[PlacementKind::kDefault];
+  };
+
+  auto placement = get_placement();
+  auto* parent = placement.parent.get();
+
+  if (parent && parent != tab_strip_region_view_->parent()) {
+    // The following remove-then-add sequence is required in order to trigger
+    // AddedToWidget in BraveTabStrip, which calls SetAvailableWidthCallback as
+    // appropriate for the current tab strip orientation.
+    if (tab_strip_region_view_->parent()) {
+      tab_strip_region_view_->parent()->RemoveChildView(
+          base::to_address(tab_strip_region_view_));
+    }
+    size_t max_index = parent->children().size();
+    size_t add_at = std::min(placement.index.value_or(max_index), max_index);
+    parent->AddChildViewAt(base::to_address(tab_strip_region_view_), add_at);
+  }
+}

browser/ui/views/frame/tab_strip_placement_coordinator.h

@@ -0,0 +1,74 @@
+/* Copyright (c) 2026 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#ifndef BRAVE_BROWSER_UI_VIEWS_FRAME_TAB_STRIP_PLACEMENT_COORDINATOR_H_
+#define BRAVE_BROWSER_UI_VIEWS_FRAME_TAB_STRIP_PLACEMENT_COORDINATOR_H_
+
+#include <optional>
+
+#include "base/containers/flat_map.h"
+#include "base/memory/raw_ptr.h"
+#include "base/memory/raw_ref.h"
+#include "base/types/pass_key.h"
+
+class BraveBrowserView;
+class BrowserWindowInterface;
+
+namespace views {
+class View;
+}  // namespace views
+
+// Coordinates parenting of the tab strip view within the browser view
+// hierarchy, which can depend upon various browser states (e.g. vertical tabs
+// or focus mode). Placement must be triggered explicitly with a call to
+// `UpdatePlacement`.
+class TabStripPlacementCoordinator {
+ public:
+  // Creates an instance for the specified browser view and tab strip region
+  // view. Instances may only be created and owned by BraveBrowserView.
+  TabStripPlacementCoordinator(base::PassKey<BraveBrowserView>,
+                               BrowserWindowInterface* browser_window_interface,
+                               views::View* tab_strip_region_view);
+
+  TabStripPlacementCoordinator(const TabStripPlacementCoordinator&) = delete;
+  TabStripPlacementCoordinator& operator=(const TabStripPlacementCoordinator&) =
+      delete;
+
+  ~TabStripPlacementCoordinator();
+
+  enum class PlacementKind {
+    kDefault = 0,
+    kTopContainer = 1,
+    kVerticalTabStrip = 2,
+  };
+
+  // Associates a placement kind with a parent and an optional child index. The
+  // specified parent view must outlive this instance, or be cleared with a call
+  // to `ClearPlacement`.
+  void SetPlacement(PlacementKind kind,
+                    views::View* parent,
+                    std::optional<size_t> index = {});
+
+  // Clears the association between a placement kind and a parent.
+  void ClearPlacement(PlacementKind kind);
+
+  // Updates the tab strip placement for the current browser state, potentially
+  // moving the view under a new parent.
+  void UpdatePlacement();
+
+ private:
+  struct Placement {
+    raw_ptr<views::View> parent = nullptr;
+    std::optional<size_t> index;
+  };
+
+  raw_ref<BrowserWindowInterface> browser_window_interface_;
+  raw_ref<views::View> tab_strip_region_view_;
+  base::flat_map<PlacementKind, Placement> placements_;
+};
+
+using TabStripPlacementKind = TabStripPlacementCoordinator::PlacementKind;
+
+#endif  // BRAVE_BROWSER_UI_VIEWS_FRAME_TAB_STRIP_PLACEMENT_COORDINATOR_H_

browser/ui/views/frame/tab_strip_placement_coordinator_browsertest.cc

@@ -0,0 +1,82 @@
+/* Copyright (c) 2026 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#include "brave/browser/ui/views/frame/tab_strip_placement_coordinator.h"
+
+#include "brave/browser/ui/tabs/brave_tab_prefs.h"
+#include "brave/browser/ui/views/frame/brave_browser_view.h"
+#include "brave/browser/ui/views/frame/vertical_tabs/vertical_tab_strip_widget_delegate_view.h"
+#include "build/build_config.h"
+#include "chrome/browser/profiles/profile.h"
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/views/frame/browser_view.h"
+#include "chrome/test/base/in_process_browser_test.h"
+#include "chrome/test/base/ui_test_utils.h"
+#include "components/prefs/pref_service.h"
+#include "content/public/test/browser_test.h"
+#include "ui/views/view.h"
+
+class TabStripPlacementCoordinatorBrowserTest : public InProcessBrowserTest {
+ protected:
+  BraveBrowserView* brave_browser_view() {
+    return BraveBrowserView::From(
+        BrowserView::GetBrowserViewForBrowser(browser()));
+  }
+
+  TabStripPlacementCoordinator* coordinator() {
+    return brave_browser_view()->tab_strip_placement_coordinator();
+  }
+
+  views::View* tab_strip_region_view() {
+    return brave_browser_view()->tab_strip_view();
+  }
+
+  views::View* tab_strip_region_view_parent() {
+    return tab_strip_region_view()->parent();
+  }
+
+  void SetVerticalTabsEnabled(bool enabled) {
+    browser()->profile()->GetPrefs()->SetBoolean(
+        brave_tabs::kVerticalTabsEnabled, enabled);
+    RunScheduledLayouts();
+  }
+};
+
+// Toggling the vertical-tabs pref reparents the tab strip into the vertical
+// tab strip's region view, then back to its original parent.
+IN_PROC_BROWSER_TEST_F(TabStripPlacementCoordinatorBrowserTest,
+                       VerticalTabsTogglesParent) {
+  views::View* const default_parent = tab_strip_region_view_parent();
+  ASSERT_TRUE(default_parent);
+
+  SetVerticalTabsEnabled(true);
+  EXPECT_NE(default_parent, tab_strip_region_view_parent());
+  EXPECT_TRUE(
+      brave_browser_view()->vertical_tab_strip_widget_delegate_view()->Contains(
+          tab_strip_region_view()));
+
+  SetVerticalTabsEnabled(false);
+  EXPECT_EQ(default_parent, tab_strip_region_view_parent());
+}
+
+// When the browser is destroyed while in vertical tabs mode, the tabstrip is
+// reparented back into its original placement such that no raw_ptrs in
+// BrowserView are left dangling.
+IN_PROC_BROWSER_TEST_F(TabStripPlacementCoordinatorBrowserTest,
+                       VerticalTabsTeardownDoesNotDangleTabStrip) {
+  browser()->profile()->GetPrefs()->SetBoolean(brave_tabs::kVerticalTabsEnabled,
+                                               true);
+
+  Browser* second = CreateBrowser(browser()->profile());
+  ASSERT_TRUE(second);
+
+  auto* second_view =
+      BraveBrowserView::From(BrowserView::GetBrowserViewForBrowser(second));
+  ASSERT_TRUE(second_view);
+  ASSERT_TRUE(second_view->vertical_tab_strip_widget_delegate_view()->Contains(
+      second_view->tab_strip_view()));
+
+  CloseBrowserSynchronously(second);
+}

browser/ui/views/frame/vertical_tabs/BUILD.gn

@@ -25,6 +25,7 @@ source_set("vertical_tabs") {
   deps = [
     "//base",
     "//brave/browser/ui:brave_tab_prefs",
+    "//brave/browser/ui/views/frame",
     "//brave/components/constants",
     "//brave/components/vector_icons",
     "//chrome/app:command_ids",

browser/ui/views/frame/vertical_tabs/vertical_tab_strip_region_view.cc

@@ -23,6 +23,7 @@
 #include "brave/browser/ui/tabs/brave_tab_prefs.h"
 #include "brave/browser/ui/views/brave_tab_search_bubble_host.h"
 #include "brave/browser/ui/views/frame/brave_browser_view.h"
+#include "brave/browser/ui/views/frame/tab_strip_placement_coordinator.h"
 #include "brave/browser/ui/views/tabs/brave_new_tab_button.h"
 #include "brave/browser/ui/views/tabs/brave_tab_search_button.h"
 #include "brave/browser/ui/views/tabs/brave_tab_strip_layout_helper.h"
@@ -267,6 +268,13 @@ class ResettableResizeArea : public views::ResizeArea {
 BEGIN_METADATA(ResettableResizeArea)
 END_METADATA
 
+TabStripPlacementCoordinator* GetPlacementCoordinator(
+    BrowserView* browser_view) {
+  CHECK(browser_view);
+  return BraveBrowserView::From(browser_view)
+      ->tab_strip_placement_coordinator();
+}
+
 }  // namespace
 
 BraveVerticalTabStripRegionView::BraveVerticalTabStripRegionView(
@@ -289,6 +297,11 @@ BraveVerticalTabStripRegionView::BraveVerticalTabStripRegionView(
   region_view_container_->SetLayoutManager(
       std::make_unique<views::FillLayout>());
 
+  auto* placement_coordinator = GetPlacementCoordinator(browser_view);
+  CHECK(placement_coordinator);
+  placement_coordinator->SetPlacement(TabStripPlacementKind::kVerticalTabStrip,
+                                      region_view_container_.get());
+
   separator_ = AddChildView(std::make_unique<views::View>());
   separator_->SetBackground(
       views::CreateSolidBackground(kColorBraveVerticalTabSeparator));
@@ -373,7 +386,10 @@ BraveVerticalTabStripRegionView::BraveVerticalTabStripRegionView(
 BraveVerticalTabStripRegionView::~BraveVerticalTabStripRegionView() {
   // We need to move tab strip region to its original parent to avoid crash
   // during drag and drop session.
-  UpdateLayout(true);
+  if (auto* coordinator = GetPlacementCoordinator(browser_view_)) {
+    coordinator->ClearPlacement(TabStripPlacementKind::kVerticalTabStrip);
+  }
+  UpdateLayout();
 }
 
 void BraveVerticalTabStripRegionView::ToggleState() {
@@ -670,7 +686,7 @@ void BraveVerticalTabStripRegionView::Layout(PassKey) {
 }
 
 void BraveVerticalTabStripRegionView::OnShowVerticalTabsPrefChanged() {
-  UpdateLayout(/* in_destruction= */ false);
+  UpdateLayout();
 
   if (!tabs::utils::ShouldShowBraveVerticalTabs(browser_) &&
       state_ == State::kFloating) {
@@ -686,34 +702,23 @@ void BraveVerticalTabStripRegionView::OnBrowserPanelsMoved() {
   PreferredSizeChanged();
 }
 
-void BraveVerticalTabStripRegionView::UpdateLayout(bool in_destruction) {
-  if (tabs::utils::ShouldShowBraveVerticalTabs(browser_) && !in_destruction) {
-    if (!Contains(original_region_view_)) {
-      original_parent_of_region_view_ = original_region_view_->parent();
-      tab_strip_region_view_original_index_ =
-          original_parent_of_region_view_->GetIndexOf(original_region_view_);
-      original_parent_of_region_view_->RemoveChildView(original_region_view_);
-      region_view_container_->AddChildView(original_region_view_.get());
-
-      // Resize area can be overlapped with tabs.
-      // To make it grabbable, it should be top-most view.
-      ReorderChildView(resize_area_, children().size() - 1);
-    }
-
-    static_cast<views::FlexLayout*>(original_region_view_->GetLayoutManager())
-        ->SetOrientation(views::LayoutOrientation::kVertical);
-  } else {
-    if (Contains(original_region_view_)) {
-      region_view_container_->RemoveChildView(original_region_view_.get());
-      CHECK(tab_strip_region_view_original_index_.has_value());
-      original_parent_of_region_view_->AddChildViewAt(
-          original_region_view_.get(), *tab_strip_region_view_original_index_);
-    }
-
-    static_cast<views::FlexLayout*>(original_region_view_->GetLayoutManager())
-        ->SetOrientation(views::LayoutOrientation::kHorizontal);
+void BraveVerticalTabStripRegionView::UpdateLayout() {
+  if (auto* coordinator = GetPlacementCoordinator(browser_view_)) {
+    coordinator->UpdatePlacement();
   }
 
+  bool vertical_tabs = tabs::utils::ShouldShowBraveVerticalTabs(browser_);
+  auto layout_orientation = vertical_tabs
+                                ? views::LayoutOrientation::kVertical
+                                : views::LayoutOrientation::kHorizontal;
+
+  if (vertical_tabs) {
+    ReorderChildView(resize_area_, children().size() - 1);
+  }
+
+  static_cast<views::FlexLayout*>(original_region_view_->GetLayoutManager())
+      ->SetOrientation(layout_orientation);
+
   UpdateNewTabButtonVisibility();
 
   PreferredSizeChanged();

browser/ui/views/frame/vertical_tabs/vertical_tab_strip_region_view.h

@@ -157,7 +157,7 @@ class BraveVerticalTabStripRegionView : public views::View,
   void OnShowVerticalTabsPrefChanged();
   void OnBrowserPanelsMoved();
 
-  void UpdateLayout(bool in_destruction = false);
+  void UpdateLayout();
 
   void OnCollapsedPrefChanged();
   void OnFloatingModePrefChanged();
@@ -190,9 +190,6 @@ class BraveVerticalTabStripRegionView : public views::View,
 
   raw_ptr<BrowserView> browser_view_ = nullptr;
   raw_ptr<Browser> browser_ = nullptr;
-
-  raw_ptr<views::View> original_parent_of_region_view_ = nullptr;
-  std::optional<size_t> tab_strip_region_view_original_index_;
   raw_ptr<HorizontalTabStripRegionView> original_region_view_ = nullptr;
 
   // Reportedly, when we add the TabStripRegionView to