wpf control width binding
change path to ViewportWidth too !
<Grid x:Name="mainGrid">//and some elements exist there <Grid> <Border x:Name="border_btn" Width="{Binding ElementName=root ,Path=ViewportWidth, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/> </Grid></Grid>
use ViewportWidth instead of ActualWidth because in this case ActualWidth is always shown 0! for test this problem you can test this with this line in your xaml.
<TextBlock Text="{Binding ElementName=mainGrid, Path=ActualWidth ,Mode=TwoWay}" Foreground="Red"/>