Scrollbars of ScrollViewer around ZoomBorder don't seem to work correctly if VerticalAlignment and HorizontalAlignment aren't set to Top/Left

[greenwaym]

If VerticalAlignment and HorizonalAlignment are set to Stretch, then the Scroll bars of the Scrollviewer surrounding the Zoomborder don't allow scrolling to the whole content, but instead the content is cut off. This doesn't happen when VerticalAlignment is set to "Top" and HorizontalAlignment is set to "Left".

Minimal Example to reproduce the issue:

<Window x:Class="PanAndZoomTests.Views.MainWindow"
        xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:paz="using:Avalonia.Controls.PanAndZoom"
        WindowStartupLocation="CenterScreen" UseLayoutRounding="True"
        Title="PanAndZoom" Height="640" Width="640">
        <ScrollViewer VerticalScrollBarVisibility="Auto"
                      HorizontalScrollBarVisibility="Auto" 
                      Background="Red">
            <paz:ZoomBorder Name="ZoomBorder" 
                            Stretch="None" 
                            ZoomSpeed="1.2"
                            Background="SlateBlue" 
                            ClipToBounds="True" 
                            Focusable="True"
                            VerticalAlignment="Stretch" 
                            HorizontalAlignment="Stretch">
                <Border Width="800" Height="600" BorderThickness="2" BorderBrush="Black" Background="Wheat"/>
                
            </paz:ZoomBorder>  
        </ScrollViewer>
</Window>

Here we just have the ZoomBorder with a simple Border control as content. When we set VerticalAlignment="Stretch" and HorizontalAlignment="Stretch"> on ZoomBorder, the Scrollbars of the ScrollViewer don't allow to correctly scroll the full content. This is visible as my border control is a black border brush in this example, thus when panning using the scroll bars, it should be possible to see the border brush on all sides. That is not the case.

Potential reason:

Is it possible that when the bounds are calculated in ZoomBorder, that the position of the content in Zoomborder is assumed to be 0,0? This is the case with Alignment set to top, left, as the ZoomBorder then just becomes the size of it's child component. The Child component seems to be centered as the ZoomBorder behaves like a border. So as soon as ZoomBorder isn't the same size as it's content, due to stretch ( or a specified size ), then the position of the child component isn't 0,0 anymore, but rather will have an offset due to the centering.

After further testing, the issue seems to be far more widespread then just regarding the Scrollbars. It seems to also effect Content Bounds Restriction with BoundsMode, and potentially any other operations that rely on correct bounds calculations.

[greenwaym]

I am working on a fix. I managed to fix the Scrollbars issue. While doing that, I found a bunch of other issues with the same root cause. I will update the issue with more context once I have a solid understanding of what is going on.

[greenwaym]

I tried two things, they both have their pros and cons. We could either set the position of the child component of zoomborder to a fixed (0,0) position in ArrangeOverride, guaranteeing that the the _elements position is always (0,0) and any translation is only done with the matrix. We would loose the possibility to use HorizontalAlignment/VerticalAlignment for any child component though.

Alternatively, we could add code to a large amount of the methods (I think it was 16 methods that needed updating. usually only a few lines of code per method) to consider any position value due to layout offset. I have a PR ready for this approach if you are interested in this approach. This way, we would keep the automatic centering of the child component, and allow the use of HorizontalAlignment/VerticalAlignment in the child component, thus, keeping the default border behaviour.