Add background color to grid to prevent animation overflow (#10716)

## Summary of the Pull Request
Add an explicit background color to part of the settings UI to prevent animation overflow. The previous solution (adding a ScrollViewer) caused problems.

## References
#10619 adds a ScrollViewer for one of the issues in #10609

## PR Checklist
* [x] Closes #10664
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

## Detailed Description of the Pull Request / Additional comments

## Validation Steps Performed
Visually confirmed the animation doesn't overflow, changed the theme and confirmed the colors are responsive. Confirmed the extra scrollbar is gone.

src/cascadia/TerminalSettingsEditor/MainPage.xaml

@@ -116,11 +116,12 @@
                 <RowDefinition Height="*" />
                 <RowDefinition Height="Auto" />
             </Grid.RowDefinitions>
-            <ScrollViewer Grid.Row="0">
-                <Frame x:Name="contentFrame" />
-            </ScrollViewer>
+            <Frame x:Name="contentFrame"
+                   Grid.Row="0" />
+            <!--  Explicitly set the background color on grid to prevent the navigation animation from overflowing it  -->
             <Grid Grid.Row="1"
                   Height="100"
+                  Background="{ThemeResource SystemAltHighColor}"
                   BorderBrush="{ThemeResource SystemBaseLowColor}"
                   BorderThickness="0,1,0,0">
                 <Grid.ColumnDefinitions>