Avoid select all text in editable ComboBox on mouse click

When clicking in a TextBox, the cursor is placed in the position where I click.

When clicking in an editable ComboBox, the entire text is selected, and I have to click again (after a few milliseconds), to place the cursor.

How can I make the ComboBox react the same way as the TextBox.

<StackPanel>
    <TextBox  Margin="10"
              Text="This is a test" />
    <ComboBox Name="combobox1" Margin="10"
              IsEditable="True"
              Text="This is a test"
               />
</StackPanel>

Calling the TextBox.SellectAll() is an intended implementation in ComboBox:

private static void OnPreviewMouseButtonDown(object sender, MouseButtonEventArgs e)
{
	ComboBox comboBox = (ComboBox)sender;

	if (comboBox.IsEditable)
	{
		Visual originalSource = e.OriginalSource as Visual;
		Visual textBox = comboBox.EditableTextBoxSite;

		if (originalSource != null && textBox != null
			&& textBox.IsAncestorOf(originalSource))
		{
			if (comboBox.IsDropDownOpen && !comboBox.StaysOpenOnEdit)
			{
				// When combobox is not editable, clicks anywhere outside
				// the combobox will close it.  When the combobox is editable
				// then clicking the text box should close the combobox as well.
				comboBox.Close();
			}
			else if (!comboBox.IsContextMenuOpen && !comboBox.IsKeyboardFocusWithin)
			{
				// If textBox is clicked, claim focus
				comboBox.Focus();
				e.Handled = true;   // Handle so that textbox won't try to update cursor position
			}
		}
	}
}
private static void OnGotFocus(object sender, RoutedEventArgs e)
{
	// When ComboBox gets logical focus, select the text inside us.
	ComboBox comboBox = (ComboBox)sender;

	// If we're an editable combobox, forward focus to the TextBox element
	if (!e.Handled)
	{
		if (comboBox.IsEditable && comboBox.EditableTextBoxSite != null)
		{
			if (e.OriginalSource == comboBox)
			{
				comboBox.EditableTextBoxSite.Focus();
				e.Handled = true;
			}
			else if (e.OriginalSource == comboBox.EditableTextBoxSite)
			{
				comboBox.EditableTextBoxSite.SelectAll();
			}
		}
	}
}

Such a way MouseDown event being stopped and ComboBox.GotFocus -> TextBox.GotFocus being initiated on mouse click.

In order to reach the behavior as by TextBox you need to subscribe for TextBox.GotFocus, you can do it in event handler for ComboBox.Loaded, and post setting the TextBox.CaretIndex to the right position.

All this you can pack to a behavior:

public class NoSelectAllOnMouseClick : Behavior<ComboBox>
{
    protected override void OnAttached()
    {
        base.OnAttached();
        AssociatedObject.Loaded += AssociatedObject_Loaded;
    }

    private void AssociatedObject_Loaded(object sender, System.Windows.RoutedEventArgs e)
    {
        var tbx = FindVisualChild<TextBox>(AssociatedObject);
        if (tbx != null)
        {
            tbx.GotFocus += Tbx_GotFocus;
        }
    }
    private async void Tbx_GotFocus(object sender, RoutedEventArgs e)
    {
        var tbx = sender as TextBox;
        if (tbx == null)
        {
            return;
        }
        var mousePos = Mouse.GetPosition(tbx);

        if (tbx.InputHitTest(mousePos) != null) //Check point is in the TextBox
        {
            await tbx.Dispatcher.InvokeAsync(() =>
            {
                tbx.CaretIndex = tbx.GetCharacterIndexFromPoint(mousePos, true) + 1;
            });
        }
    }

    protected override void OnDetaching()
    {
        AssociatedObject.Loaded -= AssociatedObject_Loaded;
        base.OnDetaching();
    }

    private childItem FindVisualChild<childItem>(DependencyObject obj) where childItem : DependencyObject
    {
        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(obj, i);
            if (child != null && child is childItem)
            {
                return (childItem)child;
            }
            else
            {
                childItem childOfChild = FindVisualChild<childItem>(child);
                if (childOfChild != null)
                {
                    return childOfChild;
                }
            }
        }
        return null;
    }
}

and use it:

<ComboBox x:Name="combobox1" Margin="10" xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
    IsEditable="True"
    Text="This is a test">
    <i:Interaction.Behaviors>
        <beh:NoSelectAllOnMouseClick/>
    </i:Interaction.Behaviors>
</ComboBox>