Make WPF Application Fullscreen (Cover startmenu)
You're probably missing the WindowState="Maximized", try the following:
<Window x:Class="HTA.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" WindowStyle="None" ResizeMode="NoResize" WindowStartupLocation="CenterScreen" WindowState="Maximized">
<Window x:Class="HTA.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" mc:Ignorable="d" ResizeMode="NoResize" WindowStartupLocation="CenterScreen" Width="1024" Height="768" WindowState="Maximized" WindowStyle="None">Window state to Maximized and window style to None