如果 UniformSpacingPanel 包含 Collapsed 元素，那么对齐行为不正确

[CodingOctocat]

Describe the bug

如果 UniformSpacingPanel 包含 Collapsed 元素，那么对齐行为不正确，如下所示，其中的 CheckBox 折叠后，右边缘多了 12 单位的内边距。

n 个折叠的元素，导致右侧产生 n * HorizontalSpacing 的内边距。

<hc:UniformSpacingPanel Grid.Row="4"
                        Margin="0,8,0,0"
                        HorizontalAlignment="Right"
                        HorizontalSpacing="12"
                        Orientation="Horizontal">
    <CheckBox Visibility="Visible/Collapsed"/>

    <Button />

    <ToggleButton />
</hc:UniformSpacingPanel>

Visible or Hidden:

Collapsed:

Expected behavior

No response

Screenshots

No response

NuGet package version

HandyControls (Custom version) 3.6.0

IDE

Visual Studio 2026

Framework type

.Net 10.0

Windows version

Windows 11

Additional context

No response

[CodingOctocat]

问题分析与修复方案：
在计算间距时没有跳过 Collapsed 的子元素，导致折叠元素虽然不占尺寸，但仍然参与了 isFirst 判断和间距累加。

protected override Size MeasureOverride(Size constraint)
{
    var curLineSize = new PanelUvSize(_orientation);
    var panelSize = new PanelUvSize(_orientation);
    var uvConstraint = new PanelUvSize(_orientation, constraint);
    var itemWidth = ItemWidth;
    var itemHeight = ItemHeight;
    var itemWidthSet = !double.IsNaN(itemWidth);
    var itemHeightSet = !double.IsNaN(itemHeight);
    var childWrapping = ChildWrapping;
    var itemHorizontalAlignment = ItemHorizontalAlignment;
    var itemVerticalAlignment = ItemVerticalAlignment;
    var itemHorizontalAlignmentSet = itemHorizontalAlignment != null;
    var itemVerticalAlignmentSet = itemVerticalAlignment != null;
    var spacingSize = GetSpacingSize();

    var childConstraint = new Size(
        itemWidthSet ? itemWidth : constraint.Width,
        itemHeightSet ? itemHeight : constraint.Height);

    var children = InternalChildren;
    var isFirst = true;

    if (childWrapping == VisualWrapping.NoWrap)
    {
        var layoutSlotSize = constraint;

        if (_orientation == Orientation.Horizontal)
        {
            layoutSlotSize.Width = double.PositiveInfinity;
        }
        else
        {
            layoutSlotSize.Height = double.PositiveInfinity;
        }

        for (int i = 0, count = children.Count; i < count; ++i)
        {
            var child = children[i];
            if (child is null) continue;
            if (child.Visibility == Visibility.Collapsed) continue; // ✅ Fix

            if (itemHorizontalAlignmentSet)
            {
                child.SetCurrentValue(HorizontalAlignmentProperty, itemHorizontalAlignment);
            }

            if (itemVerticalAlignmentSet)
            {
                child.SetCurrentValue(VerticalAlignmentProperty, itemVerticalAlignment);
            }

            child.Measure(layoutSlotSize);

            var sz = new PanelUvSize(
                _orientation,
                itemWidthSet ? itemWidth : child.DesiredSize.Width,
                itemHeightSet ? itemHeight : child.DesiredSize.Height);

            curLineSize.U += isFirst ? sz.U : sz.U + spacingSize.U;
            curLineSize.V = Math.Max(sz.V, curLineSize.V);

            isFirst = false;
        }
    }
    else
    {
        for (int i = 0, count = children.Count; i < count; i++)
        {
            var child = children[i];
            if (child is null) continue;
            if (child.Visibility == Visibility.Collapsed) continue; // ✅ Fix

            if (itemHorizontalAlignmentSet)
            {
                child.SetCurrentValue(HorizontalAlignmentProperty, itemHorizontalAlignment);
            }

            if (itemVerticalAlignmentSet)
            {
                child.SetCurrentValue(VerticalAlignmentProperty, itemVerticalAlignment);
            }

            child.Measure(childConstraint);

            var sz = new PanelUvSize(
                _orientation,
                itemWidthSet ? itemWidth : child.DesiredSize.Width,
                itemHeightSet ? itemHeight : child.DesiredSize.Height);

            if (!isFirst && MathHelper.GreaterThan(curLineSize.U + sz.U + spacingSize.U, uvConstraint.U))
            {
                panelSize.U = Math.Max(curLineSize.U, panelSize.U);
                panelSize.V += curLineSize.V + spacingSize.V;
                curLineSize = sz;
                isFirst = false; // 换行后新行的第一个可见元素，重置 isFirst
            }
            else
            {
                curLineSize.U += isFirst ? sz.U : sz.U + spacingSize.U;
                curLineSize.V = Math.Max(sz.V, curLineSize.V);
                isFirst = false;
            }
        }
    }

    panelSize.U = Math.Max(curLineSize.U, panelSize.U);
    panelSize.V += curLineSize.V;

    return new Size(panelSize.Width, panelSize.Height);
}

protected override Size ArrangeOverride(Size finalSize)
{
    var firstInLine = 0;
    var itemWidth = ItemWidth;
    var itemHeight = ItemHeight;
    double accumulatedV = 0;
    var itemU = _orientation == Orientation.Horizontal ? itemWidth : itemHeight;
    var curLineSize = new PanelUvSize(_orientation);
    var uvFinalSize = new PanelUvSize(_orientation, finalSize);
    var itemWidthSet = !double.IsNaN(itemWidth);
    var itemHeightSet = !double.IsNaN(itemHeight);
    var useItemU = _orientation == Orientation.Horizontal ? itemWidthSet : itemHeightSet;
    var childWrapping = ChildWrapping;
    var spacingSize = GetSpacingSize();

    var children = InternalChildren;
    var isFirst = true;

    if (childWrapping == VisualWrapping.NoWrap)
    {
        ArrangeLine(uvFinalSize.V, useItemU, itemU, spacingSize.U);
    }
    else
    {
        for (int i = 0, count = children.Count; i < count; i++)
        {
            var child = children[i];
            if (child is null) continue;
            if (child.Visibility == Visibility.Collapsed) continue; // ✅ Fix

            var sz = new PanelUvSize(
                _orientation,
                itemWidthSet ? itemWidth : child.DesiredSize.Width,
                itemHeightSet ? itemHeight : child.DesiredSize.Height);

            if (!isFirst && MathHelper.GreaterThan(curLineSize.U + sz.U + spacingSize.U, uvFinalSize.U))
            {
                ArrangeWrapLine(accumulatedV, curLineSize.V, firstInLine, i, useItemU, itemU, spacingSize.U);

                accumulatedV += curLineSize.V + spacingSize.V;
                curLineSize = sz;

                firstInLine = i;
                isFirst = false; // 换行后新行的第一个可见元素，重置 isFirst
            }
            else
            {
                curLineSize.U += isFirst ? sz.U : sz.U + spacingSize.U;
                curLineSize.V = Math.Max(sz.V, curLineSize.V);
                isFirst = false;
            }
        }

        if (firstInLine < children.Count)
        {
            ArrangeWrapLine(accumulatedV, curLineSize.V, firstInLine, children.Count, useItemU, itemU,
                spacingSize.U);
        }
    }

    return finalSize;
}