Fix ElasticWrapPanel not skipping hidden children when IsFillHorizontal and ItemWidth are set

[Copilot]

When IsFillHorizontal="True" and ItemWidth are set together, ElasticWrapPanel reserves layout space for invisible children. Without these properties the bug is masked because invisible children report DesiredSize of zero — but the fixed ItemWidth path bypasses that natural zero-size shortcut, assigning full slot width to hidden items regardless of visibility.

Changes

• ElasticWrapPanel.cs — Add if (!child.IsVisible) continue; at the top of the child iteration loop in both MeasureOverride and ArrangeOverride, consistently skipping invisible children before any size calculation or collection placement occurs.

foreach (var child in children)
{
    if (!child.IsVisible) continue;  // ← added
    UVSize sz;
    if (GetIsFixToRB(child)) { ... }
    else
    {
        sz = new UVSize(orientation,
            itemWidthSet ? itemWidth : child.DesiredSize.Width,  // was using itemWidth for hidden children
            ...);
    }
}

• ElasticWrapPanelTests/Tests.cs — Two new tests asserting hidden items are skipped both with and without ItemWidth/IsFillHorizontal set.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>ElasticWrapPanel控件不跳过隐藏项问题</issue_title>
<issue_description>ElasticWrapPanel控件，设置了IsFillHorizontal="True" ItemWidth="320"这两项后，隐藏的就不能跳过了，会继续占位。不设置这两个属性可以跳过隐藏项

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ElasticWrapPanel控件不跳过隐藏项问题 #895

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