Fixed contextual panel item is not activated after attaching to window

fix brave/brave-browser#41782

We called SidebarContainerView::WillShowSidePanel() to observer
tab's contextual entry/global entry when panel starts to show
from some places. But there was corner-case that WillShowSidePanel()
is not called when panel is shown. It happened when tab is attached
to another window. And realized that PopulateSidePanel() is always
called whenever panel is shown in whatever situation.
Fixed by calling WillShowSidePanel() from PopulateSidePanel().

browser/ui/sidebar/sidebar_browsertest.cc

@@ -1242,6 +1242,21 @@ IN_PROC_BROWSER_TEST_F(SidebarBrowserTestWithAIChat,
   EXPECT_TRUE(model()->active_index().has_value());
   EXPECT_EQ(model()->active_index(),
             model()->GetIndexOf(SidebarItem::BuiltInItemType::kBookmarks));
+
+  // Check tab specific panel item is still activated after moving to another
+  // browser.
+  tab_model()->ActivateTabAt(1);
+  EXPECT_EQ(model()->active_index(), tab_specific_item_index);
+
+  auto* browser2 = CreateBrowser(browser()->profile());
+  auto* browser2_model =
+      static_cast<BraveBrowser*>(browser2)->sidebar_controller()->model();
+  auto* browser2_tab_model = browser2->tab_strip_model();
+
+  auto detached_tab = tab_model()->DetachTabAtForInsertion(1);
+  browser2_tab_model->AppendTab(std::move(detached_tab), /* foreground */ true);
+  EXPECT_EQ(browser2_model->active_index(),
+            browser2_model->GetIndexOf(SidebarItem::BuiltInItemType::kChatUI));
 }
 
 IN_PROC_BROWSER_TEST_F(SidebarBrowserTestWithAIChat,

browser/ui/views/side_panel/brave_side_panel_coordinator.cc

@@ -42,11 +42,6 @@ void BraveSidePanelCoordinator::Show(
   sidebar::SetLastUsedSidePanel(browser_view_->GetProfile()->GetPrefs(),
                                 entry_key.id());
 
-  // Notify to give opportunity to observe another panel entries from
-  // global or active tab's contextual registry.
-  auto* brave_browser_view = static_cast<BraveBrowserView*>(browser_view_);
-  brave_browser_view->WillShowSidePanel();
-
   SidePanelCoordinator::Show(entry_key, open_trigger);
 }
 
@@ -91,13 +86,6 @@ void BraveSidePanelCoordinator::Toggle() {
 void BraveSidePanelCoordinator::Toggle(
     SidePanelEntryKey key,
     SidePanelUtil::SidePanelOpenTrigger open_trigger) {
-  // Notify to give opportunity to observe another panel entries from
-  // global or active tab's contextual registry.
-  if (!IsSidePanelShowing()) {
-    auto* brave_browser_view = static_cast<BraveBrowserView*>(browser_view_);
-    brave_browser_view->WillShowSidePanel();
-  }
-
   SidePanelCoordinator::Toggle(key, open_trigger);
 }
 
@@ -160,18 +148,18 @@ void BraveSidePanelCoordinator::PopulateSidePanel(
     return;
   }
 
+  // Notify to give opportunity to observe another panel entries from
+  // global or active tab's contextual registry.
+  auto* brave_browser_view = static_cast<BraveBrowserView*>(browser_view_);
+  brave_browser_view->WillShowSidePanel();
+
   SidePanelCoordinator::PopulateSidePanel(supress_animations, unique_key, entry,
                                           std::move(content_view));
 }
 
 void BraveSidePanelCoordinator::NotifyPinnedContainerOfActiveStateChange(
     SidePanelEntryKey key,
     bool is_active) {
-  // // Notify to give opportunity to observe another panel entries from
-  // // global or active tab's contextual registry.
-  // auto* brave_browser_view = static_cast<BraveBrowserView*>(browser_view_);
-  // brave_browser_view->WillShowSidePanel();
-
   if (!browser_view_->toolbar()->pinned_toolbar_actions_container()) {
     return;
   }

browser/ui/views/side_panel/brave_side_panel_coordinator.h

@@ -9,7 +9,7 @@
 #include <memory>
 #include <optional>
 
-#include "chrome/browser/ui/views/side_panel/side_panel_coordinator.h"  // IWYU pragma: export
+#include "chrome/browser/ui/views/side_panel/side_panel_coordinator.h"
 
 class BraveSidePanelCoordinator : public SidePanelCoordinator {
  public: