Hide upstream's side panel header in V2 (#35616)

Resolves https://github.com/brave/brave-browser/issues/54648

Disabled upstream's panel header by calling `SidePanelEntry::set_should_show_header(false);`.


TEST=SidebarBrowserTest.SidebarV2NoUpstreamHeaderTest

browser/ui/sidebar/sidebar_browsertest.cc

@@ -2126,6 +2126,21 @@ IN_PROC_BROWSER_TEST_F(SidebarBrowserTest, SidebarV2ActiveItemStateSync) {
       [&]() { return !model()->active_index().has_value(); }));
 }
 
+// Verify that the upstream SidePanelHeader is never added when a sidebar panel
+// is opened in V2, so Brave can render its own header.
+IN_PROC_BROWSER_TEST_F(SidebarBrowserTest, SidebarV2NoUpstreamHeaderTest) {
+  auto* panel_ui = browser()->GetFeatures().side_panel_ui();
+  auto* browser_view = BrowserView::GetBrowserViewForBrowser(browser());
+  auto* side_panel = browser_view->contents_height_side_panel();
+  side_panel->DisableAnimationsForTesting();
+
+  panel_ui->Toggle();
+  ASSERT_TRUE(base::test::RunUntil([&]() { return side_panel->GetVisible(); }));
+
+  EXPECT_EQ(nullptr, side_panel->GetHeaderView<views::View>())
+      << "Upstream SidePanelHeader should not be present after V2 panel open";
+}
+
 #endif  // BUILDFLAG(ENABLE_SIDEBAR_V2)
 
 }  // namespace sidebar

browser/ui/views/side_panel/brave_side_panel_utils.cc

@@ -36,6 +36,11 @@ void RegisterContextualSidePanel(SidePanelRegistry* registry,
         /*default_content_width_callback=*/base::NullCallback()));
   }
 #endif
+
+  // Brave has its own side panel header, so hide the built-in entry headers.
+  for (auto& entry : registry->entries()) {
+    entry->set_should_show_header(false);
+  }
 }
 
 }  // namespace brave

chromium_src/chrome/browser/ui/views/side_panel/side_panel_util.cc

@@ -45,4 +45,9 @@ void SidePanelUtil::PopulateGlobalEntries(Browser* browser,
         base::NullCallback()));
   }
 #endif
+
+  // Brave has its own side panel header, so hide the built-in entry headers.
+  for (auto& entry : global_registry->entries()) {
+    entry->set_should_show_header(false);
+  }
 }