How prevent a label in a stack layout form getting cut off

How can I prevent "42" from getting cut off?

You can set the MinimumWidthRequest in the "42" Label to prevent it from being cut off.

For details about MinimumWidthRequest please refer to remarks section of the document.

Code example:

MainPage = new ContentPage {
    Content = new StackLayout {
        Orientation = StackOrientation.Horizontal,
        VerticalOptions = LayoutOptions.CenterAndExpand,
        BackgroundColor = Color.Orange,
        Padding = 10,
        Children = {
           new Label {
              Text = "The answer to life the universe and everything The answer to life the universe and everything",
              HorizontalOptions = LayoutOptions.Start,
              LineBreakMode = LineBreakMode.TailTruncation,
              BackgroundColor = Color.Yellow,
           },
           new Label {
              MinimumWidthRequest = 50,
              Text = "42",
              HorizontalOptions = LayoutOptions.EndAndExpand,
              LineBreakMode = LineBreakMode.NoWrap,
              BackgroundColor = Color.Yellow,
           },
       },
    },
};

Here is the result:

Here you can use Grid instead of StackLayout. This will solve your problem.

Grid:

var grid = new Grid();
grid.RowDefinitions.Add(new RowDefinition { Height = GridLength.Auto });
grid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) });
grid.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Auto });