Fix initial state of custom ScrollBar

Author: nicolaihenriksen

src/MaterialDesignThemes.Wpf/Behaviors/TextBoxHorizontalScrollBarBehavior.cs

@@ -1,4 +1,4 @@
-using Microsoft.Xaml.Behaviors;
+using Microsoft.Xaml.Behaviors;
 
 namespace MaterialDesignThemes.Wpf.Behaviors;
 
@@ -29,7 +29,7 @@ private static void TargetScrollBarChanged(DependencyObject d, DependencyPropert
     }
 
     public static readonly DependencyProperty TargetScrollBarVisibilityProperty =
-        DependencyProperty.Register(nameof(TargetScrollBarVisibility), typeof(ScrollBarVisibility), typeof(TextBoxHorizontalScrollBarBehavior), new PropertyMetadata(ScrollBarVisibility.Auto));
+        DependencyProperty.Register(nameof(TargetScrollBarVisibility), typeof(ScrollBarVisibility), typeof(TextBoxHorizontalScrollBarBehavior), new PropertyMetadata(ScrollBarVisibility.Hidden));
     public ScrollBarVisibility TargetScrollBarVisibility
     {
         get => (ScrollBarVisibility)GetValue(TargetScrollBarVisibilityProperty);

src/MaterialDesignThemes.Wpf/Converters/TextBoxHorizontalScrollBarWidthConverter.cs

@@ -9,7 +9,7 @@ public class TextBoxHorizontalScrollBarWidthConverter : IMultiValueConverter
     {
         if (values is [double contentHostWidth, Visibility verticalScrollBarVisibility])
         {
-            return contentHostWidth - (verticalScrollBarVisibility == Visibility.Visible ? SystemParameters.VerticalScrollBarWidth : 0);
+            return Math.Max(0, contentHostWidth - (verticalScrollBarVisibility == Visibility.Visible ? SystemParameters.VerticalScrollBarWidth : 0));
         }
         return double.NaN;
     }

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.TextBox.xaml

@@ -177,7 +177,7 @@
                                 UseLayoutRounding="{TemplateBinding UseLayoutRounding}"
                                 VerticalScrollBarVisibility="Hidden">
                       <b:Interaction.Behaviors>
-                        <behaviors:TextBoxHorizontalScrollBarBehavior TargetScrollBar="{Binding ElementName=CustomScrollBar}" TargetScrollBarVisibility="{Binding RelativeSource={RelativeSource AncestorType=TextBox}, Path=HorizontalScrollBarVisibility}" />
+                        <behaviors:TextBoxHorizontalScrollBarBehavior TargetScrollBar="{Binding ElementName=CustomScrollBar}" TargetScrollBarVisibility="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=HorizontalScrollBarVisibility}" />
                       </b:Interaction.Behaviors>
                     </ScrollViewer>
 
@@ -307,7 +307,8 @@
                   <ScrollBar x:Name="CustomScrollBar"
                              Grid.Row="1"
                              HorizontalAlignment="Left"
-                             Orientation="Horizontal">
+                             Orientation="Horizontal"
+                             Visibility="Collapsed">
                     <ScrollBar.Margin>
                       <MultiBinding Converter="{StaticResource TextBoxHorizontalScrollBarMarginConverter}">
                         <Binding ElementName="LeadingPackIcon" Path="ActualWidth" />