[Sidebar] Suppress animation when switching between panels (#36203)

Add suppress_animations=true when ActivatePanelItem is called while a
panel is already active.

Resolves brave/brave-browser#55285

browser/ui/sidebar/sidebar_browsertest.cc

@@ -2223,6 +2223,101 @@ IN_PROC_BROWSER_TEST_F(SidebarBrowserTest,
       << "No-border resize area width should equal kNoBorderResizeAreaWidth";
 }
 
+class ScopedSidePanelUIForTesting {
+ public:
+  ScopedSidePanelUIForTesting(SidebarController* controller, SidePanelUI* ui)
+      : controller_(controller) {
+    controller_->SetSidePanelUIForTesting(ui);
+  }
+  ~ScopedSidePanelUIForTesting() {
+    controller_->SetSidePanelUIForTesting(nullptr);
+  }
+
+ private:
+  raw_ptr<SidebarController> controller_;
+};
+
+class MockSidePanelUI : public SidePanelUI {
+ public:
+  MOCK_METHOD(void,
+              Show,
+              (SidePanelEntryId, std::optional<SidePanelOpenTrigger>, bool),
+              (override));
+  MOCK_METHOD(void,
+              Show,
+              (SidePanelEntryKey, std::optional<SidePanelOpenTrigger>, bool),
+              (override));
+  MOCK_METHOD(void, ShowFrom, (SidePanelEntryKey, gfx::Rect), (override));
+  MOCK_METHOD(void,
+              Close,
+              (SidePanelEntry::PanelType, SidePanelEntryHideReason, bool),
+              (override));
+  MOCK_METHOD(void,
+              Toggle,
+              (SidePanelEntryKey, SidePanelOpenTrigger),
+              (override));
+  MOCK_METHOD(std::optional<SidePanelEntryId>,
+              GetCurrentEntryId,
+              (SidePanelEntry::PanelType),
+              (const, override));
+  MOCK_METHOD(int,
+              GetCurrentEntryDefaultContentWidth,
+              (SidePanelEntry::PanelType),
+              (const, override));
+  MOCK_METHOD(bool,
+              IsSidePanelShowing,
+              (SidePanelEntry::PanelType),
+              (const, override));
+  MOCK_METHOD(bool,
+              IsSidePanelEntryShowing,
+              (const SidePanelEntryKey&),
+              (const, override));
+  MOCK_METHOD(bool,
+              IsSidePanelEntryShowing,
+              (const SidePanelEntry::Key&, bool),
+              (const, override));
+  MOCK_METHOD(base::CallbackListSubscription,
+              RegisterSidePanelShown,
+              (SidePanelEntry::PanelType, ShownCallback),
+              (override));
+  MOCK_METHOD(void,
+              OnActiveTabChanged,
+              (content::WebContents*, content::WebContents*, bool),
+              (override));
+  MOCK_METHOD(content::WebContents*,
+              GetWebContentsForTest,
+              (SidePanelEntryId),
+              (override));
+  MOCK_METHOD(void, DisableAnimationsForTesting, (), (override));
+  MOCK_METHOD(void, SetNoDelaysForTesting, (bool), (override));
+};
+
+// Verify suppress_animations is false when opening from a closed state and
+// true when switching panels while one is already active.
+IN_PROC_BROWSER_TEST_F(SidebarBrowserTest,
+                       SidebarV2ActivatePanelItemSuppressAnimation) {
+  MockSidePanelUI mock_ui;
+  ScopedSidePanelUIForTesting scoped_ui(controller(), &mock_ui);
+
+  // No active panel: opening should animate (suppress_animations=false).
+  ASSERT_FALSE(model()->active_index().has_value())
+      << "Expected no active panel before first ActivatePanelItem call";
+  EXPECT_CALL(mock_ui,
+              Show(testing::An<SidePanelEntryId>(), testing::Eq(std::nullopt),
+                   /*suppress_animations=*/false));
+  controller()->ActivatePanelItem(SidebarItem::BuiltInItemType::kBookmarks);
+  testing::Mock::VerifyAndClearExpectations(&mock_ui);
+  controller()->UpdateActiveItemState(SidebarItem::BuiltInItemType::kBookmarks);
+
+  // Active panel present: switching panels should suppress animations.
+  ASSERT_TRUE(model()->active_index().has_value())
+      << "Expected active panel after UpdateActiveItemState";
+  EXPECT_CALL(mock_ui,
+              Show(testing::An<SidePanelEntryId>(), testing::Eq(std::nullopt),
+                   /*suppress_animations=*/true));
+  controller()->ActivatePanelItem(SidebarItem::BuiltInItemType::kReadingList);
+}
+
 #endif  // BUILDFLAG(ENABLE_SIDEBAR_V2)
 
 }  // namespace sidebar

browser/ui/sidebar/sidebar_controller.cc

@@ -10,6 +10,7 @@
 
 #include "base/check.h"
 #include "base/check_op.h"
+#include "brave/browser/ui/sidebar/buildflags/buildflags.h"
 #include "brave/browser/ui/sidebar/sidebar.h"
 #include "brave/browser/ui/sidebar/sidebar_model.h"
 #include "brave/browser/ui/sidebar/sidebar_service_factory.h"
@@ -150,17 +151,25 @@ void SidebarController::ActivateItemAt(std::optional<size_t> index,
 void SidebarController::ActivatePanelItem(
     SidebarItem::BuiltInItemType panel_item) {
   // For panel item activation, SidePanelUI is the single source of truth.
-  auto* side_panel_ui = browser_->GetFeatures().side_panel_ui();
-  if (!side_panel_ui) {
-    return;
-  }
+  auto* side_panel_ui = side_panel_ui_for_testing_
+                            ? side_panel_ui_for_testing_.get()
+                            : browser_->GetFeatures().side_panel_ui();
   CHECK(side_panel_ui);
   if (panel_item == SidebarItem::BuiltInItemType::kNone) {
     side_panel_ui->Close(SidePanelEntry::PanelType::kContent);
     return;
   }
 
+#if BUILDFLAG(ENABLE_SIDEBAR_V2)
+  // Suppress opening animation when we have active item.
+  // When opening another panel while other panel is visible,
+  // we don't need to open new panel with animation.
+  const bool suppress_animations = sidebar_model_->active_index().has_value();
+  side_panel_ui->Show(sidebar::SidePanelIdFromSideBarItemType(panel_item),
+                      /*open_trigger*/ std::nullopt, suppress_animations);
+#else
   side_panel_ui->Show(sidebar::SidePanelIdFromSideBarItemType(panel_item));
+#endif
 }
 
 void SidebarController::DeactivateCurrentPanel() {

browser/ui/sidebar/sidebar_controller.h

@@ -19,6 +19,7 @@
 class Browser;
 class GURL;
 class Profile;
+class SidePanelUI;
 class TabStripModel;
 
 namespace sidebar {
@@ -63,6 +64,10 @@ class SidebarController : public SidebarService::Observer {
   void ActivatePanelItem(SidebarItem::BuiltInItemType panel_item);
   void DeactivateCurrentPanel();
 
+  void SetSidePanelUIForTesting(SidePanelUI* side_panel_ui) {
+    side_panel_ui_for_testing_ = side_panel_ui;
+  }
+
   // If current browser doesn't have a tab for |url|, active tab will load
   // |url|. Otherwise, existing tab will be activated.
   // ShowSingletonTab() has similar functionality but it loads url in the
@@ -99,6 +104,7 @@ class SidebarController : public SidebarService::Observer {
   raw_ptr<Profile> profile_ = nullptr;
   raw_ptr<Browser> browser_ = nullptr;
   raw_ptr<Sidebar> sidebar_ = nullptr;
+  raw_ptr<SidePanelUI> side_panel_ui_for_testing_ = nullptr;
 
   std::unique_ptr<SidebarModel> sidebar_model_;
   std::unique_ptr<SidebarWebPanelController> web_panel_controller_;