C# UWP Richeditbox Zooming content is bugging

Question

So I created a RichEditBox in UWP and made a custom style to set ZoomMode="Enabled" in the Scrollviewer. After that I zoomed and some lines of the text were gone, or only the half of the text was visible.

Here a short video:

So you can see the text hiding on zoom with Ctrl + mousewheel

Does anybody know if I did anything wrong, or it is a problem of the scrollviewer?

Btw my code:

    <Page.Resources>
    <Style x:Key="RichEditBoxStyle1" TargetType="RichEditBox">
        <Setter Property="Foreground" Value="{ThemeResource TextControlForeground}"/>
        <Setter Property="Background" Value="{ThemeResource TextControlBackground}"/>
        <Setter Property="ContentLinkForegroundColor" Value="{ThemeResource ContentLinkForegroundColor}"/>
        <Setter Property="ContentLinkBackgroundColor" Value="{ThemeResource ContentLinkBackgroundColor}"/>
        <Setter Property="SelectionHighlightColor" Value="{ThemeResource TextControlSelectionHighlightColor}"/>
        <Setter Property="BorderBrush" Value="{ThemeResource TextControlBorderBrush}"/>
        <Setter Property="BorderThickness" Value="{ThemeResource TextControlBorderThemeThickness}"/>
        <Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
        <Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}"/>
        <Setter Property="ScrollViewer.HorizontalScrollMode" Value="Auto"/>
        <Setter Property="ScrollViewer.VerticalScrollMode" Value="Auto"/>
        <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
        <Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="False"/>
        <Setter Property="TextWrapping" Value="Wrap"/>
        <Setter Property="Padding" Value="{ThemeResource TextControlThemePadding}"/>
        <Setter Property="UseSystemFocusVisuals" Value="{ThemeResource IsApplicationFocusVisualKindReveal}"/>
        <Setter Property="ContextFlyout" Value="{StaticResource TextControlCommandBarContextFlyout}"/>
        <Setter Property="SelectionFlyout" Value="{StaticResource TextControlCommandBarSelectionFlyout}"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="RichEditBox">
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="*"/>
                            <RowDefinition Height="Auto"/>
                        </Grid.RowDefinitions>
                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="CommonStates">
                                <VisualState x:Name="Disabled">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="HeaderContentPresenter" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlHeaderForegroundDisabled}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundDisabled}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushDisabled}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundDisabled}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlPlaceholderForegroundDisabled}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="Normal"/>
                                <VisualState x:Name="PointerOver">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushPointerOver}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundPointerOver}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlPlaceholderForegroundPointerOver}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundPointerOver}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="Focused">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="PlaceholderTextContentPresenter" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlPlaceholderForegroundFocused}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="Background">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBackgroundFocused}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="BorderElement" Storyboard.TargetProperty="BorderBrush">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlBorderBrushFocused}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="Foreground">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="{ThemeResource TextControlForegroundFocused}"/>
                                        </ObjectAnimationUsingKeyFrames>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ContentElement" Storyboard.TargetProperty="RequestedTheme">
                                            <DiscreteObjectKeyFrame KeyTime="0" Value="Light"/>
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                        <ContentPresenter x:Name="HeaderContentPresenter" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}" FontWeight="Normal" Foreground="{ThemeResource TextControlHeaderForeground}" Margin="{ThemeResource RichEditBoxTopHeaderMargin}" Grid.Row="0" TextWrapping="Wrap" VerticalAlignment="Top" Visibility="Collapsed" x:DeferLoadStrategy="Lazy"/>
                        <Border x:Name="BorderElement" Background="{TemplateBinding Background}" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="{TemplateBinding CornerRadius}" MinHeight="{ThemeResource TextControlThemeMinHeight}" MinWidth="{ThemeResource TextControlThemeMinWidth}" Grid.RowSpan="1" Grid.Row="1"/>
                        <ScrollViewer ZoomMode="Enabled" x:Name="ContentElement" AutomationProperties.AccessibilityView="Raw" HorizontalScrollBarVisibility="{TemplateBinding ScrollViewer.HorizontalScrollBarVisibility}" HorizontalScrollMode="{TemplateBinding ScrollViewer.HorizontalScrollMode}" IsDeferredScrollingEnabled="{TemplateBinding ScrollViewer.IsDeferredScrollingEnabled}" IsHorizontalRailEnabled="{TemplateBinding ScrollViewer.IsHorizontalRailEnabled}" IsTabStop="False" IsVerticalRailEnabled="{TemplateBinding ScrollViewer.IsVerticalRailEnabled}" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" VerticalScrollMode="{TemplateBinding ScrollViewer.VerticalScrollMode}" VerticalScrollBarVisibility="{TemplateBinding ScrollViewer.VerticalScrollBarVisibility}" />
                        <TextBlock x:Name="PlaceholderTextContentPresenter" Foreground="{ThemeResource TextControlPlaceholderForeground}" IsHitTestVisible="False" Margin="{TemplateBinding BorderThickness}" Padding="{TemplateBinding Padding}" Grid.Row="1" Text="{TemplateBinding PlaceholderText}" TextWrapping="{TemplateBinding TextWrapping}" TextAlignment="{TemplateBinding TextAlignment}"/>
                        <ContentPresenter x:Name="DescriptionPresenter" AutomationProperties.AccessibilityView="Raw" Content="{TemplateBinding Description}" Foreground="{ThemeResource SystemControlDescriptionTextForegroundBrush}" Grid.Row="2" x:Load="False"/>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Page.Resources>

<Grid>
    <RichEditBox Style="{StaticResource RichEditBoxStyle1}" FontSize="24"/>
</Grid>

Answer 1

First of all, the explanation of this behavior is that the text is not gone. They are still in the Scrollviewer but out of the visible area. If you scroll the slider of the Scrollviewer, you should be able to find this text upper left corner.

But I got a different behavior when testing your code in a blank UWP app. When I tried to zoom the RichEditBox, the content of the RichEditBox is zoomed, but they are still visible because the Scrollviewer will automatically relocate itself. When the zoom action is finished, the Scrollviewer will align upper left corner. So the text should be always visible.