XAML window doesn't look like what I want - wpf

Question

I'm learning about WPF and I'm creating a window in XAML.
The window should look like this:
But when I run the program it looks like this:
The code is the following:
<Page x:Class="WpfApp1.ProductsManagement"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WpfApp1"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800"
Title="ProductsManagement">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="80" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="300" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="420" />
</Grid.RowDefinitions>
<TextBlock
Margin="5"
Text="Search"
Grid.Row="0"
Grid.Column="0"/>
<TextBox
Margin="5"
Grid.ColumnSpan="2"
Grid.Column="1"
Background ="White"
Grid.Row="0"
Text="hi"/>
<DataGrid
Margin ="5"
Name="dataGrid"
Grid.Column="0"
Grid.ColumnSpan="2"
Grid.Row="1"/>
<Border
Margin ="5"
Grid.Row="1"
Grid.Column="2"/>
</Grid>
</Page>
Any comments or suggestions are welcome.
UPDATE
I'm taking the following code as an example:
<Page x:Class="WpfApp1.Discussion"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:data="clr-namespace:BikeShop"
xmlns:local="clr-namespace:WpfApp1"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800"
Title="Discussion">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="100" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="50" />
</Grid.RowDefinitions>
<ListBox
Grid.ColumnSpan="2"
Margin="5"/>
<Button
Grid.Row="1"
Grid.Column="1"
Margin="5"
Content="Send" />
<TextBox
Grid.Row="1"
Margin="5"
Text="Type your message here" />
</Grid>
</Page>
And when I run the code it looks like this: (It works correctly)

Answer 1

Your RowDefinitions must be like this:
<Grid.RowDefinitions>
<RowDefinition Height="40"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
Right now you ask to fill the entire page for the first row and set the second row to a height of 420.
You must define a specific value for the first and * for the second.
You do not see the error in the designer because you set the second row to 420. Obviously you see the first row at 30. But when you go to fullscreen, the first row gets bigger.

Answer 2

Because your Row heights are incorrect. replace your RawDefinitions with:
<Grid.RowDefinitions>
<RowDefinition Height="40" />
<RowDefinition Height="1*" />
</Grid.RowDefinitions>