UniformGridLayout (FlowLayout): Fix issue with scrolling up creating gaps in layout

dev/Repeater/FlowLayoutAlgorithm.cpp

@@ -364,7 +364,9 @@ void FlowLayoutAlgorithm::Generate(
                 {
                     // Does not fit, wrap to the previous row
                     const auto availableSizeMinor = Minor(availableSize);
-                    MinorStart(currentBounds) = std::isfinite(availableSizeMinor) ? availableSizeMinor - Minor(desiredSize) : 0.0f;
+                    // 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.
+                    MinorStart(currentBounds) = std::isfinite(availableSizeMinor) ? availableSizeMinor - Minor(desiredSize) : MinorSize(LastExtent()) - Minor(desiredSize);
                     MajorStart(currentBounds) = lineOffset - Major(desiredSize) - static_cast<float>(lineSpacing);
 
                     if (lineNeedsReposition)

dev/Repeater/InteractionTests/RepeaterTests.cs

@@ -36,6 +36,38 @@ public static void ClassInitialize(TestContext testContext)
             TestEnvironment.Initialize(testContext);
         }
 
+        [TestMethod]
+        public void VerifyUniformGridLayoutDoesNotCreateHoles()
+        {
+            using (var setup = new TestSetupHelper("ItemsRepeater Tests"))
+            {
+                // Open page
+                FindElement.ByName("UniformGridLayoutDemo").Click();
+
+                // Scroll down
+                var scrollviewer = FindElement.ByName("RepeaterScrollViewer");
+                var repeaterHeightButton = FindElement.ByName("GetRepeaterActualHeightButton");
+                repeaterHeightButton.Click();
+                // Get actual repeater height
+                var oldActualRepeaterHeight = double.Parse(FindElement.ByName("RepeaterActualHeightLabel").GetText());
+
+                InputHelper.RotateWheel(scrollviewer, -2000);
+                Wait.ForIdle();
+
+                // Scroll back up again, users don't scroll large offsets at once but rather small chunks in succession.
+                for(int i=0;i<5;i++)
+                {
+                    InputHelper.RotateWheel(scrollviewer, 50);
+                    Wait.ForIdle();
+                }
+
+                repeaterHeightButton.Click();
+                Verify.IsTrue(Math.Abs(oldActualRepeaterHeight - 
+                    double.Parse(FindElement.ByName("RepeaterActualHeightLabel").GetText())) < 1,
+                    "Repeater heights did not match. This indicates that there are holes in layout as the repeater now needed more/less space.");
+            }
+        }
+
         public void TestCleanup()
         {
             TestCleanupHelper.Cleanup();

dev/Repeater/TestUI/RepeaterTestUIPage.xaml

@@ -85,6 +85,7 @@
             <TextBlock Text="Other Samples" Style="{ThemeResource HeaderTextBlockStyle}"/>
             <Button x:Name="defaultDemo" AutomationProperties.Name="Default Demo" >Default Demo</Button>
             <Button x:Name="basicDemo" AutomationProperties.Name="Basic Demo">Basic Demo</Button>
+            <Button x:Name="uniformGridLayoutDemo" AutomationProperties.Name="UniformGridLayoutDemo">UniformGridLayout testing</Button>
         </StackPanel>
 
     </controls:LayoutPanel>

dev/Repeater/TestUI/RepeaterTestUIPage.xaml.cs

@@ -34,6 +34,11 @@ public RepeaterTestUIPage()
                 Frame.NavigateWithoutAnimation(typeof(BasicDemo));
             };
 
+            uniformGridLayoutDemo.Click += delegate 
+            {
+                Frame.NavigateWithoutAnimation(typeof(UniformGridLayoutDemo));
+            };
+
             itemsSourceDemo.Click += delegate 
             {
                 Frame.NavigateWithoutAnimation(typeof(ElementsInItemsSourcePage));
@@ -224,6 +229,7 @@ public RepeaterTestUIPage()
                         Orientation = orientation.IsOn ? Orientation.Horizontal : Orientation.Vertical,
                     });
             };
+
         }
 
         private VirtualizingLayout GetStackLayout()

dev/Repeater/TestUI/Repeater_TestUI.projitems

@@ -108,6 +108,10 @@
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>
     </Page>
+    <Page Include="$(MSBuildThisFileDirectory)Samples\UniformGridLayoutDemo.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
   </ItemGroup>
   <ItemGroup>
     <Compile Include="$(MSBuildThisFileDirectory)RepeaterTestUIPage.xaml.cs">
@@ -199,6 +203,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)Samples\LayoutSamples\NestedLayoutSamples\StoreMockData.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Samples\AnimationSamples\DefaultElementAnimator.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Samples\ItemsSourceSamples\ResetableCollection.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Samples\UniformGridLayoutDemo.cs">
+      <DependentUpon>UniformGridLayoutDemo.xaml</DependentUpon>
+    </Compile>
   </ItemGroup>
   <ItemGroup>
     <Content Include="$(MSBuildThisFileDirectory)Images\recipe1.png" />

dev/Repeater/TestUI/Samples/UniformGridLayoutDemo.cs

@@ -0,0 +1,31 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License. See LICENSE in the project root for license information.
+
+using Microsoft.UI.Xaml.Controls;
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Linq;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using RecyclingElementFactory = Microsoft.UI.Xaml.Controls.RecyclingElementFactory;
+using SelectTemplateEventArgs = Microsoft.UI.Xaml.Controls.SelectTemplateEventArgs;
+
+namespace MUXControlsTestApp.Samples
+{
+    public sealed partial class UniformGridLayoutDemo : Page
+    {
+        public IEnumerable<int> collection;
+
+        public UniformGridLayoutDemo()
+        {
+            collection = Enumerable.Range(0, 40);
+            this.InitializeComponent();
+        }
+
+        public void GetRepeaterActualHeightButtonClick(object sender, RoutedEventArgs e)
+        {
+            RepeaterActualHeightLabel.Text = UniformGridRepeater.ActualHeight.ToString();
+        }
+    }
+}

dev/Repeater/TestUI/Samples/UniformGridLayoutDemo.xaml

@@ -0,0 +1,40 @@
+<!-- Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT License. See LICENSE in the project root for license information. -->
+<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:controls="using:Microsoft.UI.Xaml.Controls">
+
+    <Page.Resources>
+        <!-- The Layout specifications used: -->
+        <controls:UniformGridLayout x:Name="UniformGridLayout"
+                                MinRowSpacing="8" MinColumnSpacing="8"
+                                MaximumRowsOrColumns="4"/>
+
+        <DataTemplate x:Key="SimpleElementTemplate" x:DataType="x:String">
+            <Grid Background="{ThemeResource SystemControlForegroundBaseMediumLowBrush}"
+                Width="100"
+                Height="100">
+                <TextBlock Text="{Binding}" 
+                    FontSize="20"/>
+            </Grid>
+        </DataTemplate>
+    </Page.Resources>
+    <StackPanel Orientation="Horizontal">
+        <ScrollViewer HorizontalScrollBarVisibility="Auto" 
+            HorizontalScrollMode="Auto" 
+            IsVerticalScrollChainingEnabled="False"
+            AutomationProperties.Name="RepeaterScrollViewer"
+            MaxHeight="500">
+            <controls:ItemsRepeater x:Name="UniformGridRepeater"
+                ItemsSource="{x:Bind collection}"
+                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>
+</Page>