Handle contents separator visibility with SideBySide (#29126)

fix https://github.com/brave/brave-browser/issues/46193

When rounded corners feature is disabled, contents separator should
be hidden when split view is opened.

TEST=SideBySideEnabledBrowserTest.BraveMultiContentsViewTest

browser/ui/views/frame/brave_browser_view.cc

@@ -815,6 +815,18 @@ void BraveBrowserView::GetAccessiblePanes(std::vector<views::View*>* panes) {
   }
 }
 
+void BraveBrowserView::ShowSplitView() {
+  BrowserView::ShowSplitView();
+
+  UpdateContentsSeparatorVisibility();
+}
+
+void BraveBrowserView::HideSplitView() {
+  BrowserView::HideSplitView();
+
+  UpdateContentsSeparatorVisibility();
+}
+
 bool BraveBrowserView::ShouldShowWindowTitle() const {
   if (BrowserView::ShouldShowWindowTitle()) {
     return true;
@@ -881,7 +893,8 @@ void BraveBrowserView::UpdateContentsSeparatorVisibility() {
   // refers it's preferred size.
   // Don't show that separator as split view has border around contents
   // container.
-  if (split_view_ && split_view_->IsSplitViewActive()) {
+  if ((split_view_ && split_view_->IsSplitViewActive()) ||
+      (multi_contents_view_ && multi_contents_view_->IsInSplitView())) {
     contents_separator_->SetPreferredSize({});
     return;
   }

browser/ui/views/frame/brave_browser_view.h

@@ -160,6 +160,8 @@ class BraveBrowserView : public BrowserView,
   FRIEND_TEST_ALL_PREFIXES(SpeedReaderBrowserTest, ToolbarLangs);
   FRIEND_TEST_ALL_PREFIXES(VerticalTabStripBrowserTest, ExpandedState);
   FRIEND_TEST_ALL_PREFIXES(VerticalTabStripBrowserTest, ExpandedWidth);
+  FRIEND_TEST_ALL_PREFIXES(SideBySideEnabledBrowserTest,
+                           BraveMultiContentsViewTest);
 
   static void SetDownloadConfirmReturnForTesting(bool allow);
 
@@ -180,6 +182,8 @@ class BraveBrowserView : public BrowserView,
                                  bool update_devtools_web_contents) override;
   void OnWidgetActivationChanged(views::Widget* widget, bool active) override;
   void GetAccessiblePanes(std::vector<views::View*>* panes) override;
+  void ShowSplitView() override;
+  void HideSplitView() override;
 
   void StopTabCycling();
   void UpdateSearchTabsButtonState();
@@ -203,6 +207,10 @@ class BraveBrowserView : public BrowserView,
 
   void UpdateSideBarHorizontalAlignment();
 
+  views::View* contents_separator_for_testing() const {
+    return contents_separator_;
+  }
+
   std::unique_ptr<views::Widget> vertical_tab_strip_widget_;
 
   bool closing_confirm_dialog_activated_ = false;

browser/ui/views/split_view/split_view_browsertest.cc

@@ -8,6 +8,7 @@
 #include <utility>
 
 #include "base/test/run_until.h"
+#include "brave/browser/brave_browser_features.h"
 #include "brave/browser/ui/browser_commands.h"
 #include "brave/browser/ui/tabs/brave_tab_layout_constants.h"
 #include "brave/browser/ui/tabs/features.h"
@@ -58,7 +59,8 @@ class SideBySideEnabledBrowserTest : public InProcessBrowserTest {
  public:
   SideBySideEnabledBrowserTest() {
     scoped_features_.InitWithFeatures(
-        /*enabled_features*/ {features::kSideBySide}, {});
+        /*enabled_features*/ {features::kSideBySide},
+        /*disabled_features*/ {features::kBraveWebViewRoundedCorners});
   }
   ~SideBySideEnabledBrowserTest() override = default;
 
@@ -73,13 +75,22 @@ IN_PROC_BROWSER_TEST_F(SideBySideEnabledBrowserTest,
   auto* split_view_data = browser()->GetFeatures().split_view_browser_data();
   EXPECT_FALSE(!!split_view_data);
 
+  auto* browser_view =
+      BraveBrowserView::From(BrowserView::GetBrowserViewForBrowser(browser()));
   auto* multi_contents_view = static_cast<BraveMultiContentsView*>(
-      BrowserView::GetBrowserViewForBrowser(browser())
-          ->multi_contents_view_for_testing());
+      browser_view->multi_contents_view_for_testing());
   ASSERT_TRUE(multi_contents_view);
 
+  // separator should not be empty when split view is closed.
+  EXPECT_NE(gfx::Size(),
+            browser_view->contents_separator_for_testing()->GetPreferredSize());
+
   chrome::NewSplitTab(browser());
 
+  // separator should be empty when split view is opened.
+  EXPECT_EQ(gfx::Size(),
+            browser_view->contents_separator_for_testing()->GetPreferredSize());
+
   // Check corner radius.
   auto* start_contents_web_view =
       multi_contents_view->start_contents_view_for_testing();

chromium_src/chrome/browser/ui/views/frame/browser_view.h

@@ -50,9 +50,13 @@
 #undef LoadAccelerators
 #endif
 #define LoadAccelerators virtual LoadAccelerators
+#define ShowSplitView virtual ShowSplitView
+#define HideSplitView virtual HideSplitView
 
 #include "src/chrome/browser/ui/views/frame/browser_view.h"  // IWYU pragma: export
 
+#undef HideSplitView
+#undef ShowSplitView
 #undef LoadAccelerators
 #if BUILDFLAG(IS_WIN)
 // #pragma pop_macro("LoadAccelerators")