GradientBrush in CompositionCustomVisualHandler results in wrong rendering

[llfab]

Describe the bug
I created a composition animation using a CompositionCustomVisualHandler like in ControlCatalog. I basically draw a rectangle with a gradient brush. When I use an X-offset in DrawRectangle the gradient always stays static in the the original / wrong position. Only when I use a push transform and then draw the rectangle without offset, the visualization is correct.

To Reproduce
Steps to reproduce the behavior:

1. Load the sample app in VS: https://github.com/llfab/Samples/tree/main/AvaloniaAnimationTest
2. Run like it is => correct
3. Comment / uncomment the drawing commands => incorrect

Expected behavior
I would expect the gradient brush to "move" with the rectangle and render correctly in both ways of the sample app.

Videos
Correct:
https://user-images.githubusercontent.com/31031996/230961029-061456fd-c544-4fb8-9eac-81c7a7c2ff24.mp4

Incorrect:
https://user-images.githubusercontent.com/31031996/230960984-1b1b7309-70d0-4a97-979c-34b8deb4d9a3.mp4

Desktop (please complete the following information):

• OS: Windows 10
• Version: 11.0.0-preview6

Additional context
Please find this portion of code in the sample app:

            public override void OnRender(ImmediateDrawingContext drawingContext)
            {
                if (_running)
                {
                    if (_lastServerTime.HasValue)
                    {
                        _animationElapsed += (CompositionNow.TotalMilliseconds - _lastServerTime.Value);
                        _animationElapsed %= _animationLength;
                    }
                    
                    _lastServerTime = CompositionNow.TotalMilliseconds;
                }

                double borderWidth = EffectiveSize.X;
                double borderHeight = EffectiveSize.Y;
                double indicatorWidth = 256;
                double animationStage = _animationElapsed / _animationLength;

                double offsetX = (borderWidth + indicatorWidth) * animationStage - indicatorWidth;

                // Gradient brush only works using push transform
                // if used without push transform and Rect with
                // offset instead, the rendering is wrong. The gradient seems
                // to be stuck at the original position.
                using (drawingContext.PushPreTransform(Matrix.CreateTranslation(offsetX, 0)))
                {
                    drawingContext.DrawRectangle(_brush, null, new Rect(0, 0, indicatorWidth, borderHeight));
                }

                // uncomment below to get the wrong rendering
                //drawingContext.DrawRectangle(_brush, null, new Rect(offsetX, 0, indicatorWidth, borderHeight));
            }

[kekekeks]

Working as intended, brush coordinates are relative to the current (0,0), not to the rectangle origin.

[kekekeks]

See also: https://skia.org/docs/user/coordinates/

[llfab]

Ok. I understand. So, in the upper sample, where I used 0 and 1 as relative coordinates for the brush, would push transform be the way to go or would you rather create the brush with absolute coordinates including the rect x-offset?

[kekekeks]

Actually, I think it should depend on the RelativeUnit passed to the gradient stop.

@grokys