Port microsoft/microsoft-ui-xaml#3393

ModernWpf.Controls/Repeater/Layouts/FlowLayout/FlowLayoutAlgorithm.cs

@@ -355,7 +355,9 @@ private void Generate(
                         {
                             // Does not fit, wrap to the previous row
                             var availableSizeMinor = OM.Minor(availableSize);
-                            OM.SetMinorStart(ref currentBounds, !double.IsInfinity(availableSizeMinor) ? availableSizeMinor - OM.Minor(desiredSize) : 0.0);
+                            // If the last available size is finite, start from end and subtract our desired size.
+                            // Otherwise, look at the last extent and use that for positioning.
+                            OM.SetMinorStart(ref currentBounds, !double.IsInfinity(availableSizeMinor) ? availableSizeMinor - OM.Minor(desiredSize) : OM.MinorSize(LastExtent) - OM.Minor(desiredSize));
                             OM.SetMajorStart(ref currentBounds, lineOffset - OM.Major(desiredSize) - lineSpacing);
 
                             if (lineNeedsReposition)

test/ModernWpfTestApp/RepeaterTestUIPage.xaml

@@ -186,6 +186,7 @@
                 AutomationProperties.Name="Basic Demo">
                 Basic Demo
             </Button>
+            <Button x:Name="uniformGridLayoutDemo" AutomationProperties.Name="UniformGridLayoutDemo">UniformGridLayout testing</Button>
         </StackPanel>
 
     </controls:LayoutPanel>

test/ModernWpfTestApp/RepeaterTestUIPage.xaml.cs

@@ -35,6 +35,11 @@ public RepeaterTestUIPage()
                 Frame.NavigateWithoutAnimation(typeof(BasicDemo));
             };
 
+            uniformGridLayoutDemo.Click += delegate
+            {
+                Frame.NavigateWithoutAnimation(typeof(UniformGridLayoutDemo));
+            };
+
             itemsSourceDemo.Click += delegate
             {
                 Frame.NavigateWithoutAnimation(typeof(ElementsInItemsSourcePage));

test/ModernWpfTestApp/Samples/UniformGridLayoutDemo.xaml

@@ -0,0 +1,39 @@
+<!-- Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT License. See LICENSE in the project root for license information. -->
+<ui:Page
+    x:Class="MUXControlsTestApp.Samples.UniformGridLayoutDemo"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:ui="http://schemas.modernwpf.com/2019"
+    xmlns:controls="clr-namespace:ModernWpf.Controls;assembly=ModernWpf.Controls"
+    xmlns:sys="clr-namespace:System;assembly=mscorlib">
+
+    <Page.Resources>
+        <!-- The Layout specifications used: -->
+        <controls:UniformGridLayout x:Key="UniformGridLayout"
+                                MinRowSpacing="8" MinColumnSpacing="8"
+                                MaximumRowsOrColumns="4"/>
+
+        <DataTemplate x:Key="SimpleElementTemplate" DataType="sys:String">
+            <Grid Background="{ThemeResource SystemControlForegroundBaseMediumLowBrush}"
+                Width="100"
+                Height="100">
+                <TextBlock Text="{Binding}" 
+                    FontSize="20"/>
+            </Grid>
+        </DataTemplate>
+    </Page.Resources>
+    <StackPanel Orientation="Horizontal">
+        <ScrollViewer HorizontalScrollBarVisibility="Auto" 
+            AutomationProperties.Name="RepeaterScrollViewer"
+            MaxHeight="500">
+            <controls:ItemsRepeater x:Name="UniformGridRepeater"
+                Layout="{StaticResource UniformGridLayout}"
+                ItemTemplate="{StaticResource SimpleElementTemplate}"/>
+        </ScrollViewer>
+        <StackPanel>
+            <Button AutomationProperties.Name="GetRepeaterActualHeightButton"
+                    Click="GetRepeaterActualHeightButtonClick">Get actual Repeater height</Button>
+            <TextBlock x:Name="RepeaterActualHeightLabel" AutomationProperties.Name="RepeaterActualHeightLabel"/>
+        </StackPanel>
+    </StackPanel>
+</ui:Page>

test/ModernWpfTestApp/Samples/UniformGridLayoutDemo.xaml.cs

@@ -0,0 +1,26 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License. See LICENSE in the project root for license information.
+
+using System.Collections.Generic;
+using System.Linq;
+using System.Windows;
+
+namespace MUXControlsTestApp.Samples
+{
+    public sealed partial class UniformGridLayoutDemo
+    {
+        public IEnumerable<int> collection;
+
+        public UniformGridLayoutDemo()
+        {
+            collection = Enumerable.Range(0, 40);
+            this.InitializeComponent();
+            UniformGridRepeater.ItemsSource = collection;
+        }
+
+        public void GetRepeaterActualHeightButtonClick(object sender, RoutedEventArgs e)
+        {
+            RepeaterActualHeightLabel.Text = UniformGridRepeater.ActualHeight.ToString();
+        }
+    }
+}