Make the Pane inactive border respect the theme.window.applicationTheme (#14486)

We couldn't do this before, because `App::Current().Resources().Lookup()` would always return the OS theme version of a resource. That thread has lengthy details on why.

FORTUNATELY FOR US, this isn't the first time we've dealt with this. We've come up with a workaround in the past, and we can just use it again here. 

Closes #3917

This will also make #3061 easy 😉

src/cascadia/TerminalApp/App.xaml

@@ -153,7 +153,7 @@
                                              Color="#2e2e2e" />
 
                             <StaticResource x:Key="UnfocusedBorderBrush"
-                                            ResourceKey="ApplicationPageBackgroundThemeBrush" />
+                                            ResourceKey="TabViewBackground" />
 
                             <SolidColorBrush x:Key="SettingsUiTabBrush"
                                              Color="#0c0c0c" />
@@ -169,7 +169,7 @@
                                              Color="#e8e8e8" />
 
                             <StaticResource x:Key="UnfocusedBorderBrush"
-                                            ResourceKey="ApplicationPageBackgroundThemeBrush" />
+                                            ResourceKey="TabViewBackground" />
 
                             <SolidColorBrush x:Key="SettingsUiTabBrush"
                                              Color="#ffffff" />

src/cascadia/TerminalApp/Pane.cpp

@@ -5,6 +5,8 @@
 #include "Pane.h"
 #include "AppLogic.h"
 
+#include "Utils.h"
+
 #include <Mmsystem.h>
 
 using namespace winrt::Windows::Foundation;
@@ -46,14 +48,6 @@ Pane::Pane(const Profile& profile, const TermControl& control, const bool lastFo
     _warningBellToken = _control.WarningBell({ this, &Pane::_ControlWarningBellHandler });
     _closeTerminalRequestedToken = _control.CloseTerminalRequested({ this, &Pane::_CloseTerminalRequestedHandler });
 
-    // On the first Pane's creation, lookup resources we'll use to theme the
-    // Pane, including the brushed to use for the focused/unfocused border
-    // color.
-    if (s_focusedBorderBrush == nullptr || s_unfocusedBorderBrush == nullptr)
-    {
-        _SetupResources();
-    }
-
     // Register an event with the control to have it inform us when it gains focus.
     _gotFocusRevoker = _control.GotFocus(winrt::auto_revoke, { this, &Pane::_ControlGotFocusHandler });
     _lostFocusRevoker = _control.LostFocus(winrt::auto_revoke, { this, &Pane::_ControlLostFocusHandler });
@@ -3104,16 +3098,16 @@ float Pane::_ClampSplitPosition(const bool widthOrHeight, const float requestedV
 //   * The Brush we'll use for inactive Panes - TabViewBackground (to match the
 //     color of the titlebar)
 // Arguments:
-// - <none>
+// - requestedTheme: this should be the currently active Theme for the app
 // Return Value:
 // - <none>
-void Pane::_SetupResources()
+void Pane::SetupResources(const winrt::Windows::UI::Xaml::ElementTheme& requestedTheme)
 {
     const auto res = Application::Current().Resources();
     const auto accentColorKey = winrt::box_value(L"SystemAccentColor");
     if (res.HasKey(accentColorKey))
     {
-        const auto colorFromResources = res.Lookup(accentColorKey);
+        const auto colorFromResources = ThemeLookup(res, requestedTheme, accentColorKey);
         // If SystemAccentColor is _not_ a Color for some reason, use
         // Transparent as the color, so we don't do this process again on
         // the next pane (by leaving s_focusedBorderBrush nullptr)
@@ -3131,7 +3125,10 @@ void Pane::_SetupResources()
     const auto unfocusedBorderBrushKey = winrt::box_value(L"UnfocusedBorderBrush");
     if (res.HasKey(unfocusedBorderBrushKey))
     {
-        auto obj = res.Lookup(unfocusedBorderBrushKey);
+        // MAKE SURE TO USE ThemeLookup, so that we get the correct resource for
+        // the requestedTheme, not just the value from the resources (which
+        // might not respect the settings' requested theme)
+        auto obj = ThemeLookup(res, requestedTheme, unfocusedBorderBrushKey);
         s_unfocusedBorderBrush = obj.try_as<winrt::Windows::UI::Xaml::Media::SolidColorBrush>();
     }
     else

src/cascadia/TerminalApp/Pane.h

@@ -136,6 +136,8 @@ public:
 
     bool ContainsReadOnly() const;
 
+    static void SetupResources(const winrt::Windows::UI::Xaml::ElementTheme& requestedTheme);
+
     // Method Description:
     // - A helper method for ad-hoc recursion on a pane tree. Walks the pane
     //   tree, calling a function on each pane in a depth-first pattern.
@@ -339,8 +341,6 @@ private:
         return false;
     }
 
-    static void _SetupResources();
-
     struct PanePoint
     {
         float x;

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -4175,6 +4175,22 @@ namespace winrt::TerminalApp::implementation
         const auto theme = _settings.GlobalSettings().CurrentTheme();
         auto requestedTheme{ theme.RequestedTheme() };
 
+        {
+            // Update the brushes that Pane's use...
+            Pane::SetupResources(requestedTheme);
+            // ... then trigger a visual update for all the pane borders to
+            // apply the new ones.
+            for (const auto& tab : _tabs)
+            {
+                if (auto terminalTab{ _GetTerminalTabImpl(tab) })
+                {
+                    terminalTab->GetRootPane()->WalkTree([&](auto&& pane) {
+                        pane->UpdateVisuals();
+                    });
+                }
+            }
+        }
+
         const auto res = Application::Current().Resources();
 
         // Use our helper to lookup the theme-aware version of the resource.