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How to remove frames, borders around objects in a wpf project

Here on the screens, when I hover the cursor over the object, a gray border appears around itenter image description here
Here
Here
<mah:MetroWindow xmlns:mah="http://metro.mahapps.com/winfx/xaml/controls" x:Class="WpfApp1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApp1"
mc:Ignorable="d"
FontFamily="Century Gothic"
Title="Book Library" Height="660" Width="1250" >
<Window.Resources>
<DataTemplate x:Key="ItemTemplate">
<WrapPanel>
<Image Width="200" Height="300" Stretch="Fill" Source="{Binding}"/>
<TextBlock Text="{Binding}"/>
</WrapPanel>
</DataTemplate>
</Window.Resources>
</mah:MetroWindow>

Try setting its borderthickness to 0.

WPF DockPanel not showing bottom component

I'm new to WPF and I'm trying to create a Window with two text boxes, the RichTextBox at the top, which uses most of the available space, and a TextBox at the bottom. My issue is the TextBox at the bottom is not showing. I have it at the bottom of the DockPanel. What am I missing?
Where is the XAML:
<Window x:Class="WpfApp1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApp1"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="750" MinHeight="400" MinWidth="600" Background="Black">
<DockPanel
Margin="5"
Height="Auto"
Width="Auto">
<RichTextBox
Name="richTB"
IsEnabled="True"
VerticalScrollBarVisibility="Visible"
IsReadOnly="True">
<FlowDocument
Name="flowDoc"
PagePadding="0">
<Paragraph>
<Run Text="Test" Foreground="Red"></Run>
<Run Text="Foo" Foreground="Blue"></Run>
</Paragraph>
</FlowDocument>
</RichTextBox>
<TextBox
Name="textBox"
KeyDown="EnterPressed"
Background="Plum"
DockPanel.Dock="Bottom">
</TextBox>
</DockPanel>
</Window>

You may have to reorder the children of the DockPanel.
The last child fill uses the Last child in the list of children, it isn't based on where the children are laid out within the DockPanel.
<Window x:Class="WpfApp1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApp1"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="750" MinHeight="400" MinWidth="600" Background="Black">
<DockPanel
Margin="5"
Height="Auto"
Width="Auto">
<TextBox
Name="textBox"
KeyDown="EnterPressed"
Background="Plum"
DockPanel.Dock="Bottom">
</TextBox>
<RichTextBox
Name="richTB"
IsEnabled="True"
VerticalScrollBarVisibility="Visible"
IsReadOnly="True">
<FlowDocument
Name="flowDoc"
PagePadding="0">
<Paragraph>
<Run Text="Test" Foreground="Red"></Run>
<Run Text="Foo" Foreground="Blue"></Run>
</Paragraph>
</FlowDocument>
</RichTextBox>
</DockPanel>
</Window>

You textbox has no content. The dockpanel sizes its components to the needed size, but because it has no content it has no visible size.
Place some text in it and it will show.

Windows 10 universal app UserControl in stackpanel

I need to place a custom UserControl into a Stackpannel.
I have this UserControl:
<UserControl
x:Class="ScannerApp.Custom_Controls.LocationAndQuantity"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:ScannerApp.Custom_Controls"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
d:DesignHeight="20"
d:DesignWidth="400">
<Grid Background="White">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100*"/>
<ColumnDefinition Width="80*"/>
<ColumnDefinition Width="100*"/>
</Grid.ColumnDefinitions>
<Border x:Name="border" Background="Red" BorderThickness="1" HorizontalAlignment="Left" Height="20" VerticalAlignment="Top" Width="143">
<TextBlock x:Name="locationTxt" Text="location" HorizontalAlignment="Center"></TextBlock>
</Border>
<TextBlock x:Name="quantityTxt" Text="quantity" Grid.Column="2" HorizontalAlignment="Center" TextWrapping="Wrap" VerticalAlignment="Top"/>
</Grid>
</UserControl>
and a page with stackpanel
<Page
x:Class="ScannerApp.FindPN___STEP2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:ScannerApp"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
...
<StackPanel>
<!--here I want to place the userControls-->
</StackPanel>
</Grid>
</Page>
I tried some sollutions like <controls: ...> //this could not be found by intellisense even or
<my:UserControlName Grid.Column="2" Grid.Row="2" ... />
<Window ...
xmlns:my="clr-namespace:AssemblyName"
...
/>
but I don't have the Window here... I tried to place something similar into the Page, but I don't really know what to type in there.

As said in comments, you've to fix your XAML. If you want to use custom controls you've to tell the compiler where the controls come from.
In case you Controls namespace is
ScannerApp.Custom_Controls
You've to write the Page XAML as
<Page
x:Class="ScannerApp.FindPN___STEP2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:ScannerApp.Custom_Controls" <!--FIXED HERE-->
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
...
<StackPanel>
<local:NameOfYourControl x:Name="MyNewControl" /> <!--Properties can be added-->
</StackPanel>
</Grid>
</Page>

how to hide items behind a panel in wpf

i used the following code for control display
<Window x:Class="WpfApplication29.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid >
<Border Margin="100" BorderThickness="3" BorderBrush="Black">
<Canvas>
<Label Content="This is test" FontSize="129" Width="400" Height="200"/>
</Canvas>
</Border>
</Grid>
</Window>
I want to display it as the 2nd one which is cut from below if its size is greater then the parent control or if margin is negative

You just need to use ClipToBounds="True" properties for your border
<Grid >
<Border Margin="100" BorderThickness="3" BorderBrush="Black" ClipToBounds="True">
<Canvas >
<Label Content="This is test" FontSize="129" Width="400" Height="200"/>
</Canvas>
</Border>
</Grid>

Just use ClipToBounds property of Canvas to True
<Window x:Class="WpfApplication29.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid >
<Border Margin="100" BorderThickness="3" BorderBrush="Black">
<Canvas ClipToBounds="True">
<Label Content="This is test" FontSize="129" Width="400" Height="200"/>
</Canvas>
</Border>
</Grid>
and you get the Result like this

Only see the first referenced UserControl

How it is possible that only the first UserControl ni the WindowsFormsHost is shown in the GUI while the seconde one is still hidden?
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:wf_zoom0="clr-namespace:LowLightGrab;assembly=LowLightGrab"
xmlns:wf_zoom1="clr-namespace:LowLightGrab;assembly=LowLightGrab"
Title="MainWindow" Height="350" Width="525">
<Grid>
<WindowsFormsHost Height="154" HorizontalAlignment="Left" Name="wf_zoom0" VerticalAlignment="Top" Width="215" >
<wf_zoom0:UserControl1/>
</WindowsFormsHost>
<WindowsFormsHost Height="161" HorizontalAlignment="Left" Margin="251,138,0,0" Name="wf_zoom1" VerticalAlignment="Top" Width="223" >
<wf_zoom1:UserControl1/>
</WindowsFormsHost>
</Grid>

Other than removing the margin as Eran added, You should add Rows to the Grid and place each of the elements in its own row, something lile this:
<Window x:Class="ComboboxRectangles.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:wf_zoom0="clr-namespace:LowLightGrab;assembly=LowLightGrab"
xmlns:wf_zoom1="clr-namespace:LowLightGrab;assembly=LowLightGrab"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition></ColumnDefinition>
<ColumnDefinition></ColumnDefinition>
</Grid.RowDefinitions>
<WindowsFormsHost Grid.Column="0" Height="154" HorizontalAlignment="Left" Name="wf_zoom0" VerticalAlignment="Top" Width="215" >
<wf_zoom0:UserControl1/>
</WindowsFormsHost>
<WindowsFormsHost Grid.Column="1" Height="161" HorizontalAlignment="Left" Margin="251,138,0,0" Name="wf_zoom1" VerticalAlignment="Top" Width="223" >
<wf_zoom1:UserControl1/>
</WindowsFormsHost>
</Grid>
</Window>