Fixed top separator is visible in fullscreen (#34744)

Resolves brave/brave-browser#53555

- GetTopSeparatorType(): return kNone when toolbar and bookmark bar are
  not visible so no separator is drawn at the top of the contents view
  in fullscreen.
- chromium_src: define SetDelegateForTesting (declared but not defined
  upstream) so tests can inject a layout delegate.

TEST=BraveBrowserViewTabbedLayoutImplTest.GetTopSeparatorTypeNoneWhenNoVisibleTopUI

browser/ui/views/frame/BUILD.gn

@@ -31,6 +31,18 @@ source_set("frame") {
   ]
 }
 
+source_set("unit_tests") {
+  testonly = true
+  sources = [ "layout/brave_browser_view_tabbed_layout_impl_unittest.cc" ]
+  deps = [
+    ":frame",
+    "//base",
+    "//chrome/browser/ui",
+    "//testing/gmock",
+    "//testing/gtest",
+  ]
+}
+
 source_set("browser_tests") {
   testonly = true
   defines = [ "HAS_OUT_OF_PROC_TEST_RUNNER" ]

browser/ui/views/frame/layout/brave_browser_view_tabbed_layout_impl.cc

@@ -240,6 +240,13 @@ void BraveBrowserViewTabbedLayoutImpl::DoPostLayoutVisualAdjustments(
 
 BrowserViewTabbedLayoutImpl::TopSeparatorType
 BraveBrowserViewTabbedLayoutImpl::GetTopSeparatorType() const {
+  // Return kNone when there is no visible top UI (toolbar and bookmark bar).
+  // This fixes a 1px visible separator at the top of the contents view when in
+  // browser fullscreen, where the top chrome is hidden.
+  if (!delegate().IsToolbarVisible() && !delegate().IsBookmarkBarVisible()) {
+    return TopSeparatorType::kNone;
+  }
+
   // Get the upstream separator type as a starting point. The top separator is
   // a visual line that divides the browser's top UI (toolbar, tabs) from the
   // main content area below it.

browser/ui/views/frame/layout/brave_browser_view_tabbed_layout_impl_unittest.cc

@@ -0,0 +1,101 @@
+/* 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/layout/brave_browser_view_tabbed_layout_impl.h"
+
+#include <memory>
+
+#include "chrome/browser/ui/views/frame/layout/browser_view_layout.h"
+#include "chrome/browser/ui/views/frame/layout/browser_view_layout_delegate.h"
+#include "chrome/browser/ui/views/frame/layout/browser_view_layout_params.h"
+#include "testing/gmock/include/gmock/gmock.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+// Minimal fake for BrowserViewLayoutDelegate methods that are not exercised by
+// GetTopSeparatorType() on the "no top UI" path (Brave returns early).
+class FakeBrowserViewLayoutDelegate : public BrowserViewLayoutDelegate {
+ public:
+  FakeBrowserViewLayoutDelegate() = default;
+  ~FakeBrowserViewLayoutDelegate() override = default;
+
+  bool ShouldDrawTabStrip() const override { return false; }
+  bool ShouldUseTouchableTabstrip() const override { return false; }
+  bool ShouldDrawVerticalTabStrip() const override { return false; }
+  bool IsVerticalTabStripCollapsed() const override { return false; }
+  bool ShouldDrawWebAppFrameToolbar() const override { return false; }
+  bool GetBorderlessModeEnabled() const override { return false; }
+  BrowserLayoutParams GetBrowserLayoutParams(bool) const override {
+    return BrowserLayoutParams();
+  }
+  WindowState GetBrowserWindowState() const override {
+    return WindowState::kNormal;
+  }
+  views::LayoutAlignment GetWindowTitleAlignment() const override {
+    return views::LayoutAlignment::kStart;
+  }
+  bool IsToolbarVisible() const override { return false; }
+  bool IsBookmarkBarVisible() const override { return false; }
+  bool IsInfobarVisible() const override { return false; }
+  bool IsContentsSeparatorEnabled() const override { return false; }
+  bool IsActiveTabSplit() const override { return false; }
+  bool IsActiveTabAtLeadingWindowEdge() const override { return false; }
+  const ImmersiveModeController* GetImmersiveModeController() const override {
+    return nullptr;
+  }
+  ExclusiveAccessBubbleViews* GetExclusiveAccessBubble() const override {
+    return nullptr;
+  }
+  bool IsTopControlsSlideBehaviorEnabled() const override { return false; }
+  float GetTopControlsSlideBehaviorShownRatio() const override { return 0.0f; }
+  gfx::NativeView GetHostViewForAnchoring() const override {
+    return gfx::NativeView();
+  }
+  bool HasFindBarController() const override { return false; }
+  void MoveWindowForFindBarIfNecessary() const override {}
+  bool IsWindowControlsOverlayEnabled() const override { return false; }
+  void UpdateWindowControlsOverlay(const gfx::Rect&) override {}
+  bool ShouldLayoutTabStrip() const override { return false; }
+  int GetExtraInfobarOffset() const override { return 0; }
+
+  bool ShouldShowVerticalTabs() const override { return false; }
+  bool IsVerticalTabOnRight() const override { return false; }
+  bool ShouldUseBraveWebViewRoundedCornersForContents() const override {
+    return false;
+  }
+  int GetRoundedCornersWebViewMargin() const override { return 0; }
+  bool IsBookmarkBarOnByPref() const override { return false; }
+  bool IsContentTypeSidePanelVisible() const override { return false; }
+  bool IsFullscreenForBrowser() const override { return false; }
+  bool IsFullscreenForTab() const override { return false; }
+  bool IsFullscreen() const override { return false; }
+};
+
+// Only the two visibility hooks matter for GetTopSeparatorType(); gmock makes
+// the test spell that out with EXPECT_CALL.
+class MockBrowserViewLayoutDelegate : public FakeBrowserViewLayoutDelegate {
+ public:
+  MOCK_METHOD(bool, IsToolbarVisible, (), (const, override));
+  MOCK_METHOD(bool, IsBookmarkBarVisible, (), (const, override));
+};
+
+}  // namespace
+
+TEST(BraveBrowserViewTabbedLayoutImplTest,
+     GetTopSeparatorTypeNoneWhenNoVisibleTopUI) {
+  auto mock =
+      std::make_unique<testing::NiceMock<MockBrowserViewLayoutDelegate>>();
+  EXPECT_CALL(*mock, IsToolbarVisible()).WillOnce(::testing::Return(false));
+  EXPECT_CALL(*mock, IsBookmarkBarVisible()).WillOnce(::testing::Return(false));
+
+  BrowserViewLayoutViews views;
+  auto layout = std::make_unique<BraveBrowserViewTabbedLayoutImpl>(
+      std::move(mock), nullptr, std::move(views));
+
+  // TopSeparatorType::kNone (0) - no top separator when top UI is hidden.
+  // TopSeparatorType is private member.
+  EXPECT_EQ(0, static_cast<int>(layout->GetTopSeparatorType()));
+}

chromium_src/chrome/browser/ui/views/frame/layout/browser_view_layout.cc

@@ -12,3 +12,10 @@
 #include <chrome/browser/ui/views/frame/layout/browser_view_layout.cc>
 
 #undef BrowserViewTabbedLayoutImpl
+
+// It's upstream method. Declared in header but not defined in upstream.
+// Define it here so we can use it in tests.
+void BrowserViewLayout::SetDelegateForTesting(
+    std::unique_ptr<BrowserViewLayoutDelegate> delegate) {
+  delegate_ = std::move(delegate);
+}

test/BUILD.gn

@@ -485,6 +485,7 @@ test("brave_unit_tests") {
       "//brave/browser/ui/tabs/test:unit_tests",
       "//brave/browser/ui/toolbar:brave_app_menu_unit_test",
       "//brave/browser/ui/views/download/bubble:unit_tests",
+      "//brave/browser/ui/views/frame:unit_tests",
       "//brave/browser/ui/views/infobars:unit_tests",
       "//brave/browser/ui/views/page_action:unit_tests",
       "//brave/browser/ui/views/tabs:unit_tests",