Fix: RelativePanel calculation error with extensible and aligned child

[msojocs]

What does the pull request do?

Fix the problem:

The DataGrid's scroll bar does not display correctly.

What is the current behavior?

Can not see the last item of DataGrid and horizontal scrollbar.

Decrease width, all scroll bar are lost.

What is the updated/expected behavior with this PR?

1. Create a MVVM sample
2. Modify MainWindow.axaml and MainWindowViewModel.cs as follwing code:

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:vm="using:AvaloniaBugDemo.ViewModels"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="1200" d:DesignHeight="800"
        x:Class="AvaloniaBugDemo.Views.MainWindow"
        x:DataType="vm:MainWindowViewModel"
        Icon="/Assets/avalonia-logo.ico"
        Title="AvaloniaBugDemo">
    <Design.DataContext>
        <!-- This only sets the DataContext for the previewer in an IDE,
             to set the actual DataContext for runtime, set the DataContext property in code (look at App.axaml.cs) -->
        <vm:MainWindowViewModel/>
    </Design.DataContext>
  <RelativePanel x:Name="TestRelativePanel">
    <Panel
          x:Name="Area1"
          RelativePanel.AlignTopWithPanel="True"
          RelativePanel.AlignLeftWithPanel="True"
          RelativePanel.AlignRightWithPanel="True"
          Height="100"
          Background="LightSkyBlue">
      <TextBlock
          Text="Area1"
          HorizontalAlignment="Center"
          VerticalAlignment="Center"
          />
      <!-- <Button Click="Button_OnClick">Second</Button> -->
    </Panel>
    <Panel
      x:Name="Area2"
      RelativePanel.Below="Area1"
      RelativePanel.AlignLeftWithPanel="True"
      RelativePanel.AlignBottomWithPanel="True"
      Background="DeepSkyBlue"
        Width="100"
      >
      <TextBlock
        Text="Area2"
        HorizontalAlignment="Center"
        VerticalAlignment="Center"
        Height="100"
      />
    </Panel>
    <DataGrid
        x:Name="TestGrid"
        Background="Aqua"
        ItemsSource="{Binding DataGridExample}"
        RelativePanel.Below="Area1"
        RelativePanel.RightOf="Area2"
        RelativePanel.AlignRightWithPanel="True"
        RelativePanel.AlignBottomWithPanel="True"
        IsReadOnly="True"
        Margin="0 0 0 0"
        >
      <DataGrid.Columns>
        <DataGridTextColumn Header="Id1" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id2" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id3" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id4" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id5" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id6" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id7" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id8" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id9" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
        <DataGridTextColumn Header="Id10" Width="100" Binding="{Binding Id}"></DataGridTextColumn>
      </DataGrid.Columns>
    </DataGrid>
  </RelativePanel>
</Window>

using System;
using System.Linq;
using Avalonia.Collections;

namespace AvaloniaBugDemo.ViewModels;

public partial class MainWindowViewModel : ViewModelBase
{
    public string Greeting { get; } = "Welcome to Avalonia!";
    public AvaloniaList<TestItem> DataGridExample { get; } = new (Enumerable.Range(1001, 100).Select(e => new TestItem($"{e}")));

    public MainWindowViewModel()
    {
        Console.WriteLine($"Count: {DataGridExample.Count}");
    }

}

public class TestItem(string id)
{
    public string Id { get; set; } = id;
}

How was the solution implemented (if it's not obvious)?

Original logic

1. Measure the size of DataGrid;
   In this step, availableHeight's value is wrong. availableHeight = TotalHeight = Area1.Height + RemainArea.Height
2. Calculate DataGrid's position.

Midified logic

1. Calculate DataGrid's position;
   availableHeight's value is right. availableHeight = TotalHeight - Area1.Height = RemainArea.Height
2. Measure the size of DataGrid.

Other thing

I modified the code in the branch release/11.3.4 and cherry-picked the commit to the master branch;

because I can not run a program on master branch, I do not know how to solve the compile exception.

Result

Checklist

• Added unit tests (if possible)? I tried, but failed. Due to DataGrid package build exception.
• Added XML documentation to any related classes? (I don't think it's necessary)
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation (I don't think it's necessary)

Breaking changes

No.

Obsoletions / Deprecations

No.

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0058390-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[msojocs]

@cla-avalonia agree

[MrJul]

Thank you for your contribution!

This PR needs a matching unit test, failing before the changes and passing after.
I recommend using a standard element such as a Border as the child for that test.

[msojocs]

OK, I will try it.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0058398-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0058400-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM, thank you!