How to make WPF wrappanel child items to stretch?

As another solution (sometimes width might be different, so UniformGrid won’t work), here is modified WrapPanel which would stretch elements to fit lines (while keeping proportion the same):

public class StretchyWrapPanel : Panel {
    public static readonly DependencyProperty ItemWidthProperty = DependencyProperty.Register(nameof(ItemWidth),
            typeof(double), typeof(StretchyWrapPanel), new FrameworkPropertyMetadata(double.NaN, FrameworkPropertyMetadataOptions.AffectsMeasure));

    [TypeConverter(typeof(LengthConverter))]
    public double ItemWidth {
        get { return (double)GetValue(ItemWidthProperty); }
        set { SetValue(ItemWidthProperty, value); }
    }

    public static readonly DependencyProperty ItemHeightProperty = DependencyProperty.Register(nameof(ItemHeight),
            typeof(double), typeof(StretchyWrapPanel), new FrameworkPropertyMetadata(double.NaN, FrameworkPropertyMetadataOptions.AffectsMeasure));

    [TypeConverter(typeof(LengthConverter))]
    public double ItemHeight {
        get { return (double)GetValue(ItemHeightProperty); }
        set { SetValue(ItemHeightProperty, value); }
    }

    public static readonly DependencyProperty OrientationProperty = StackPanel.OrientationProperty.AddOwner(typeof(StretchyWrapPanel),
            new FrameworkPropertyMetadata(Orientation.Horizontal, FrameworkPropertyMetadataOptions.AffectsMeasure, OnOrientationChanged));

    public Orientation Orientation {
        get { return _orientation; }
        set { SetValue(OrientationProperty, value); }
    }

    private static void OnOrientationChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
        ((StretchyWrapPanel)d)._orientation = (Orientation)e.NewValue;
    }

    private Orientation _orientation = Orientation.Horizontal;

    public static readonly DependencyProperty StretchProportionallyProperty = DependencyProperty.Register(nameof(StretchProportionally), typeof(bool),
            typeof(StretchyWrapPanel), new PropertyMetadata(true, OnStretchProportionallyChanged));

    public bool StretchProportionally {
        get { return _stretchProportionally; }
        set { SetValue(StretchProportionallyProperty, value); }
    }

    private static void OnStretchProportionallyChanged(DependencyObject o, DependencyPropertyChangedEventArgs e) {
        ((StretchyWrapPanel)o)._stretchProportionally = (bool)e.NewValue;
    }

    private bool _stretchProportionally = true;

    private struct UVSize {
        internal UVSize(Orientation orientation, double width, double height) {
            U = V = 0d;
            _orientation = orientation;
            Width = width;
            Height = height;
        }

        internal UVSize(Orientation orientation) {
            U = V = 0d;
            _orientation = orientation;
        }

        internal double U;
        internal double V;
        private readonly Orientation _orientation;

        internal double Width {
            get { return _orientation == Orientation.Horizontal ? U : V; }
            set {
                if (_orientation == Orientation.Horizontal) {
                    U = value;
                } else {
                    V = value;
                }
            }
        }

        internal double Height {
            get { return _orientation == Orientation.Horizontal ? V : U; }
            set {
                if (_orientation == Orientation.Horizontal) {
                    V = value;
                } else {
                    U = value;
                }
            }
        }
    }

    protected override Size MeasureOverride(Size constraint) {
        var curLineSize = new UVSize(Orientation);
        var panelSize = new UVSize(Orientation);
        var uvConstraint = new UVSize(Orientation, constraint.Width, constraint.Height);
        var itemWidth = ItemWidth;
        var itemHeight = ItemHeight;
        var itemWidthSet = !double.IsNaN(itemWidth);
        var itemHeightSet = !double.IsNaN(itemHeight);

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

        var children = InternalChildren;

        for (int i = 0, count = children.Count; i < count; i++) {
            var child = children[i];
            if (child == null) continue;

            // Flow passes its own constrint to children
            child.Measure(childConstraint);

            // This is the size of the child in UV space
            var sz = new UVSize(Orientation,
                    itemWidthSet ? itemWidth : child.DesiredSize.Width,
                    itemHeightSet ? itemHeight : child.DesiredSize.Height);

            if (curLineSize.U + sz.U > uvConstraint.U) {
                // Need to switch to another line
                panelSize.U = Math.Max(curLineSize.U, panelSize.U);
                panelSize.V += curLineSize.V;
                curLineSize = sz;

                if (sz.U > uvConstraint.U) {
                    // The element is wider then the constrint - give it a separate line             
                    panelSize.U = Math.Max(sz.U, panelSize.U);
                    panelSize.V += sz.V;
                    curLineSize = new UVSize(Orientation);
                }
            } else {
                // Continue to accumulate a line
                curLineSize.U += sz.U;
                curLineSize.V = Math.Max(sz.V, curLineSize.V);
            }
        }

        // The last line size, if any should be added
        panelSize.U = Math.Max(curLineSize.U, panelSize.U);
        panelSize.V += curLineSize.V;

        // Go from UV space to W/H space
        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 UVSize(Orientation);
        var uvFinalSize = new UVSize(Orientation, finalSize.Width, finalSize.Height);
        var itemWidthSet = !double.IsNaN(itemWidth);
        var itemHeightSet = !double.IsNaN(itemHeight);
        var useItemU = Orientation == Orientation.Horizontal ? itemWidthSet : itemHeightSet;

        var children = InternalChildren;

        for (int i = 0, count = children.Count; i < count; i++) {
            var child = children[i];
            if (child == null) continue;

            var sz = new UVSize(Orientation, itemWidthSet ? itemWidth : child.DesiredSize.Width, itemHeightSet ? itemHeight : child.DesiredSize.Height);
            if (curLineSize.U + sz.U > uvFinalSize.U) {
                // Need to switch to another line
                if (!useItemU && StretchProportionally) {
                    ArrangeLineProportionally(accumulatedV, curLineSize.V, firstInLine, i, uvFinalSize.Width);
                } else {
                    ArrangeLine(accumulatedV, curLineSize.V, firstInLine, i, true, useItemU ? itemU : uvFinalSize.Width / Math.Max(1, i - firstInLine - 1));
                }

                accumulatedV += curLineSize.V;
                curLineSize = sz;

                if (sz.U > uvFinalSize.U) {
                    // The element is wider then the constraint - give it a separate line     
                    // Switch to next line which only contain one element
                    if (!useItemU && StretchProportionally) {
                        ArrangeLineProportionally(accumulatedV, sz.V, i, ++i, uvFinalSize.Width);
                    } else {
                        ArrangeLine(accumulatedV, sz.V, i, ++i, true, useItemU ? itemU : uvFinalSize.Width);
                    }

                    accumulatedV += sz.V;
                    curLineSize = new UVSize(Orientation);
                }
                firstInLine = i;
            } else {
                // Continue to accumulate a line
                curLineSize.U += sz.U;
                curLineSize.V = Math.Max(sz.V, curLineSize.V);
            }
        }

        // Arrange the last line, if any
        if (firstInLine < children.Count) {
            if (!useItemU && StretchProportionally) {
                ArrangeLineProportionally(accumulatedV, curLineSize.V, firstInLine, children.Count, uvFinalSize.Width);
            } else {
                ArrangeLine(accumulatedV, curLineSize.V, firstInLine, children.Count, true,
                        useItemU ? itemU : uvFinalSize.Width / Math.Max(1, children.Count - firstInLine - 1));
            }
        }

        return finalSize;
    }

    private void ArrangeLineProportionally(double v, double lineV, int start, int end, double limitU) {
        var u = 0d;
        var horizontal = Orientation == Orientation.Horizontal;
        var children = InternalChildren;

        var total = 0d;
        for (var i = start; i < end; i++) {
            total += horizontal ? children[i].DesiredSize.Width : children[i].DesiredSize.Height;
        }

        var uMultipler = limitU / total;
        for (var i = start; i < end; i++) {
            var child = children[i];
            if (child != null) {
                var childSize = new UVSize(Orientation, child.DesiredSize.Width, child.DesiredSize.Height);
                var layoutSlotU = childSize.U * uMultipler;
                child.Arrange(new Rect(horizontal ? u : v, horizontal ? v : u,
                        horizontal ? layoutSlotU : lineV, horizontal ? lineV : layoutSlotU));
                u += layoutSlotU;
            }
        }
    }

    private void ArrangeLine(double v, double lineV, int start, int end, bool useItemU, double itemU) {
        var u = 0d;
        var horizontal = Orientation == Orientation.Horizontal;
        var children = InternalChildren;
        for (var i = start; i < end; i++) {
            var child = children[i];
            if (child != null) {
                var childSize = new UVSize(Orientation, child.DesiredSize.Width, child.DesiredSize.Height);
                var layoutSlotU = useItemU ? itemU : childSize.U;
                child.Arrange(new Rect(horizontal ? u : v, horizontal ? v : u,
                        horizontal ? layoutSlotU : lineV, horizontal ? lineV : layoutSlotU));
                u += layoutSlotU;
            }
        }
    }
}

In action:

To achieve the desired behavior you may try to use Width or MaxWidth properties of WrapPanel, for example by this way

        <ListView
            Name="RootControl"
            ItemsSource="{Binding [icons]}"
            ItemTemplate="{StaticResource IconTemplate}">
            <ListView.ItemsPanel>
                <ItemsPanelTemplate>
                    <WrapPanel
                        Width="{Binding ElementName=RootControl, Path=ActualWidth}"/>
                </ItemsPanelTemplate>
            </ListView.ItemsPanel>
        </ListView>

It works

Use a UniformGrid rather than a WrapPanel. Just set the number of columns you want with the Columns property, and it should give you the desired result.