Fix settings error text legibility in High Contrast mode (#20098)

Closes #18147

`SystemErrorTextColor` doesn't adapt to High Contrast themes, so the
error details in the "Failed to reload settings" dialog are illegible
(yellow on cream in HC White, for example).

Fix: skip the `ErrorTextBrush` foreground override when High Contrast is
active, letting the text inherit the system HC text color from its
parent element. This matches the existing HC detection pattern used in
`TermControl.cpp` and `MainPage.cpp`.

Validated manually on High Contrast White and Night sky themes.

Co-authored-by: SushaanthSrinivasan <SushaanthSrinivasan@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/cascadia/TerminalApp/AppActionHandlers.cpp

@@ -499,8 +499,8 @@ namespace winrt::TerminalApp::implementation
             }
             else
             {
-                _ResizePane(realArgs.ResizeDirection());
-                args.Handled(true);
+                const auto resizeSucceeded = _ResizePane(realArgs.ResizeDirection());
+                args.Handled(resizeSucceeded);
             }
         }
     }

src/cascadia/TerminalApp/Tab.cpp

@@ -844,14 +844,14 @@ namespace winrt::TerminalApp::implementation
     // Arguments:
     // - direction: The direction to move the separator in.
     // Return Value:
-    // - <none>
-    void Tab::ResizePane(const ResizeDirection& direction)
+    // - whether a pane was resized
+    bool Tab::ResizePane(const ResizeDirection& direction)
     {
         ASSERT_UI_THREAD();
 
         // NOTE: This _must_ be called on the root pane, so that it can propagate
         // throughout the entire tree.
-        _rootPane->ResizePane(direction);
+        return _rootPane->ResizePane(direction);
     }
 
     // Method Description:

src/cascadia/TerminalApp/Tab.h

@@ -53,7 +53,7 @@ namespace winrt::TerminalApp::implementation
                                                                                                         const float splitSize,
                                                                                                         winrt::Windows::Foundation::Size availableSpace) const;
 
-        void ResizePane(const winrt::Microsoft::Terminal::Settings::Model::ResizeDirection& direction);
+        bool ResizePane(const winrt::Microsoft::Terminal::Settings::Model::ResizeDirection& direction);
         bool NavigateFocus(const winrt::Microsoft::Terminal::Settings::Model::FocusDirection& direction);
         bool SwapPane(const winrt::Microsoft::Terminal::Settings::Model::FocusDirection& direction);
         bool FocusPane(const uint32_t id);

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -2900,14 +2900,15 @@ namespace winrt::TerminalApp::implementation
     // Arguments:
     // - direction: The direction to move the separator in.
     // Return Value:
-    // - <none>
-    void TerminalPage::_ResizePane(const ResizeDirection& direction)
+    // - whether a pane was resized
+    bool TerminalPage::_ResizePane(const ResizeDirection& direction)
     {
         if (const auto tabImpl{ _GetFocusedTabImpl() })
         {
             _UnZoomIfNeeded();
-            tabImpl->ResizePane(direction);
+            return tabImpl->ResizePane(direction);
         }
+        return false;
     }
 
     // Method Description:

src/cascadia/TerminalApp/TerminalPage.h

@@ -426,7 +426,7 @@ namespace winrt::TerminalApp::implementation
                         const Microsoft::Terminal::Settings::Model::SplitDirection splitType,
                         const float splitSize,
                         std::shared_ptr<Pane> newPane);
-        void _ResizePane(const Microsoft::Terminal::Settings::Model::ResizeDirection& direction);
+        bool _ResizePane(const Microsoft::Terminal::Settings::Model::ResizeDirection& direction);
         void _ToggleSplitOrientation();
 
         void _ScrollPage(ScrollDirection scrollDirection);

src/cascadia/TerminalApp/TerminalWindow.cpp

@@ -108,13 +108,19 @@ static Documents::Run _BuildErrorRun(const winrt::hstring& text, const ResourceD
     Documents::Run textRun;
     textRun.Text(text);
 
-    // Color the text red (light theme) or yellow (dark theme) based on the system theme
-    auto key = winrt::box_value(L"ErrorTextBrush");
-    if (resources.HasKey(key))
+    // GH #18147 - In High Contrast mode, don't override the foreground.
+    // Let the text inherit the system HC text color from its parent element,
+    // since SystemErrorTextColor doesn't adapt to High Contrast themes.
+    if (!winrt::Windows::UI::ViewManagement::AccessibilitySettings{}.HighContrast())
     {
-        auto g = resources.Lookup(key);
-        auto brush = g.try_as<winrt::Windows::UI::Xaml::Media::Brush>();
-        textRun.Foreground(brush);
+        // Color the text red (light theme) or yellow (dark theme) based on the system theme
+        auto key = winrt::box_value(L"ErrorTextBrush");
+        if (resources.HasKey(key))
+        {
+            auto g = resources.Lookup(key);
+            auto brush = g.try_as<winrt::Windows::UI::Xaml::Media::Brush>();
+            textRun.Foreground(brush);
+        }
     }
 
     return textRun;

src/cascadia/WindowsTerminal/WindowEmperor.cpp

@@ -99,9 +99,8 @@ static const uint8_t* deserializeString(const uint8_t* it, const uint8_t* end, w
     uint32_t len;
     it = deserializeUint32(it, end, len);
 
-    const auto bytes = static_cast<size_t>(len) * sizeof(wchar_t);
-
-    if (bytes == 0 || static_cast<size_t>(end - it) < bytes)
+    size_t bytes{};
+    if (!SUCCEEDED(SizeTMult(static_cast<size_t>(len), sizeof(wchar_t), &bytes)) || bytes == 0 || static_cast<size_t>(end - it) < bytes)
     {
         throw std::out_of_range("Not enough data for string content");
     }