[Proposal]: Should the `Element.Measure` method modify the `DesiredSize` returned by the subclass?

[chenjing1294]

Problem statement

Should the Element.Measure method modify the DesiredSize returned by the subclass? As far as I know, in WPF and AvaloniaUI, DesiredSize is not modified; it's limited during the Arrange phase.

So, is it possible to modify it like this?

// src/MewUI/Elements/Element.cs
public void Measure(Size availableSize)
{
  if (!IsMeasureDirty && _hasMeasureConstraint && _lastMeasureConstraint == availableSize)
  {
      return;
  }

  Size measured;
  using (PerformanceProfiler.Instance.SampleElement(GetType(), ProfilerSampleCategory.Measure))
  {
      measured = MeasureCore(availableSize);
  }

  /*var clamped = new Size(
      double.IsPositiveInfinity(availableSize.Width)
          ? measured.Width
          : Math.Min(measured.Width, availableSize.Width),
      double.IsPositiveInfinity(availableSize.Height)
          ? measured.Height
          : Math.Min(measured.Height, availableSize.Height));

  DesiredSize = ApplyLayoutRounding(clamped);*/

  DesiredSize = ApplyLayoutRounding(measured);
  IsMeasureDirty = false;
  _lastMeasureConstraint = availableSize;
  _hasMeasureConstraint = true;
}

// src/MewUI/Elements/FrameworkElement.cs
protected override Rect GetArrangedBounds(Rect finalRect)
{
  var innerSlot = finalRect.Deflate(Margin);

  double availableWidth = Math.Max(0, innerSlot.Width);
  double availableHeight = Math.Max(0, innerSlot.Height);

  /*double arrangeWidth = !double.IsNaN(Width) ? Width : DesiredSize.Width - Margin.Left - Margin.Right;
    double arrangeHeight = !double.IsNaN(Height) ? Height : DesiredSize.Height - Margin.Top - Margin.Bottom;*/
  double arrangeWidth;
  double arrangeHeight;
  // If we have explicit size, use it; otherwise use desired (excluding margin)
  if (!double.IsNaN(Width))
  {
      arrangeWidth = Width;
  }
  else
  {
      arrangeWidth = DesiredSize.Width - Margin.HorizontalThickness;
      arrangeWidth = Math.Min(availableWidth, arrangeWidth);
  }
  
  if (!double.IsNaN(Height))
  {
      arrangeHeight = Height;
  }
  else
  {
      arrangeHeight = DesiredSize.Height - Margin.VerticalThickness;
      arrangeHeight = Math.Min(availableHeight, arrangeHeight);
  }

Why the current approach is insufficient

see above

Type of change

Behavior change

Expected impact

see above

Alternatives or tradeoffs

see above

Additional context

No response

Before submitting

• I searched for existing issues and discussions first.
• I described why this belongs in MewUI core rather than only in app-level code.

[al6uiz]

Thanks for continuing to report layout issues. Your previous reports were also helpful.

I think the concern raised here is valid. However, measure/arrange behavior can affect many areas, including panels, scrolling, and fit-to-content behavior, so I will review this carefully together with regression tests.

I will use your proposal as a reference while looking for the appropriate fix.