Same here, very annoying.
I've reverted to 11.2.4.

In 11.2.4, the problem also appears when using browser [wasm] app.

It's likely that the PR #18310 had some unintended effect here.

cc @xoofx @Gillibald

Y I think I will revert that PR and introduce a different fix. WidthIncludingTrailingWhitespace is also broken in 11.2.4. It does not produce the correct results.

With Courier New

 <StackPanel VerticalAlignment="Center" HorizontalAlignment="Center">
   <TextBlock HorizontalAlignment="Center" VerticalAlignment="Center" Background="Magenta" FontSize="44" Text="aaaa"/>
   <TextBlock Margin="0 10 0 0" HorizontalAlignment="Center" VerticalAlignment="Center" Background="Magenta" FontSize="44" Text="a a "/>
   <TextBlock Margin="0 10 0 0" HorizontalAlignment="Center" VerticalAlignment="Center" Background="Magenta" FontSize="44" Text="    "/>
 </StackPanel>

This is the expected result.

This is with current Avalonia

I would prefer that we start from the current code that is more readable and fixed another annoying issue and fix it from there. As the repro case above should be easy to add as a unit test, I can have a look right now if that's ok?

Feel free to have a look

Here is my fix:

        private void UpdateMetrics(TextLineImpl currentLine, ref bool first)
        {
            _metrics.InkBounds = _metrics.InkBounds.Union(new Rect(new Point(0, _metrics.Bounds.Bottom) + currentLine.InkBounds.Position, currentLine.InkBounds.Size));
            _metrics.Bounds = _metrics.Bounds.Union(new Rect(new Point(0, _metrics.Bounds.Bottom) + currentLine.Bounds.Position, currentLine.Bounds.Size));

            _metrics.MinTextWidth = Math.Max(_metrics.MinTextWidth, currentLine.Bounds.Width);
            _metrics.MinTextWidth = Math.Max(_metrics.MinTextWidth, currentLine.InkBounds.Width);

            _metrics.Height = _metrics.Bounds.Height;
            _metrics.Width = _metrics.InkBounds.Width;
            _metrics.WidthIncludingTrailingWhitespace = Math.Max(_metrics.WidthIncludingTrailingWhitespace, _metrics.Bounds.Left + currentLine.WidthIncludingTrailingWhitespace);
            _metrics.Extent = _metrics.InkBounds.Height;
            _metrics.OverhangLeading = Math.Max(0, _metrics.Bounds.Left - _metrics.InkBounds.Left);
            _metrics.OverhangTrailing = Math.Max(0, _metrics.InkBounds.Right - _metrics.Bounds.Right);
            _metrics.OverhangAfter = Math.Max(0, _metrics.InkBounds.Bottom - _metrics.Bounds.Bottom);

            // 6) Capture the baseline from the first line.
            if (first)
            {
                _metrics.Baseline = currentLine.Baseline;
                first = false;
            }
        }

This is the previous implementation with a fix for the WidthIncludingTrailingWhitespace issue.

Compare it to your version, and you will realise that all overhang values have been calculated incorrectly. Extend, OverhandLeading, OverhangTrailing and OverhangAfter represent the black pixels drawn outside the logical bounds. Your implementation just builds the overall maximum.

Created a fix via #18438

Compare it to your version, and you will realise that all overhang values have been calculated incorrectly. Extend, OverhandLeading, OverhangTrailing and OverhangAfter represent the black pixels drawn outside the logical bounds. Your implementation just builds the overall maximum.

I don't think that the previous code was correct either regarding these. We should reflect the OverhandLeading/Trailing/After given by the actual line metrics and not recompute something that has no link to the original line metrics.

The problem was with MinTextWidth that was previously taking into account the WidthIncludingTrailingWhitespace, but the previous code was hiding this through bounds, which is not desirable because we don't understand what is in the box (e.g. the Width of Bounds from the LineMetrics is actually WidthIncludingTrailingWhitespace)

Yes, the metrics were never correct. Each line reports its metrics relative to the line's origin. Each line can have a different origin. So you can't just take the maximum OverhangLeading etc. You need to calculate this value in respect to the ink and logical bounding box.