Use size enforced by Width/Height for content measurements

[kekekeks]

If Size is being enforced by Width/Height, contents are still measured with availableSize.

Minimal code to reproduce the issue:

<Window xmlns="https://github.com/avaloniaui" Title="Test app" WindowState="Maximized" MinWidth="500" MinHeight="300">
    <Button Content="Button text" Margin="40,65,0,0" Width="201" Height="65">
        <TextBlock>Button Text</TextBlock>
    </Button>
</Window>

I'm not sure that we should fix it this way, so no unit test for now

[grokys]

This should already be handled by this line:

Avalonia/src/Avalonia.Layout/LayoutHelper.cs

Line 24 in 760caf8

double width = (control.Width > 0) ? control.Width : constraints.Width;

I've written a unit test to test the behavior, and it passes:

        [Fact]
        public void Width_Should_Affect_AvailableSize()
        {
            MeasureTest target;

            var outer = new Decorator
            {
                Width = 100,
                Height = 100,
                Child = target = new MeasureTest
                {
                    Width = 70,
                }
            };

            outer.Measure(Size.Infinity);

            Assert.Equal(new Size(70, 100), target.AvailableSize);
        }

        class MeasureTest : Control
        {
            public Size? AvailableSize { get; private set; }

            protected override Size MeasureOverride(Size availableSize)
            {
                AvailableSize = availableSize;
                return availableSize;
            }
        }

In the example you gave, the width and height properties also seem to be getting applied correctly before MeasureOverride. Maybe your problem is the fact that the margin is applied after the layout constraints are applied?

It might help in this situation to write failing unit tests first, to give us an idea of what the problem might be.

[kekekeks]

Fixed by #994