ItemsRepeater renders only the first item when no ScrollViewer in the visual tree under a popup

Describe the bug If there is no ScrollViewer in the parent visual tree of an ItemsRepeater, then it will render only the first item.

Steps to reproduce the bug

1. Create a blank app, in the MainPage put:

MainPage.xaml

<Grid>
	<Popup x:Name="popup" IsOpen="True">
		<controls:ItemsRepeater x:Name="SUT" />
	</Popup>
</Grid>

MainPage.xaml.cs

SUT.ItemsSource = Enumerable.Range(0, 10);

Expected behavior As the Popup is tall enough, we should see all 10 items.

Screenshots

Version Info

• Microsoft.UI.Xaml: 2.4.0-prerelease.200322001
• Windows: 1909 18363.778_
• Project’s target and min version: 18362
• Device: Desktop

Additional context According to my tests, this is because in the ViewportManagerWithPlatformFeatures, if we cannot find any scroller we init the viewport with an empty rect. This drives the GetLayoutRealizationWindow to return an empty Rect. This “window” is used by the FlowLayoutAlgorithm.ShouldContinueFillingUpSpace(), which concludes that the viewport/window is full and stops the elements generation after the first one.

Note: this is not visible out of a Popup as a normal Window does have a ScrollViewer in its “template”.

[dr1rrb]

Hi @ranjeshj , yes it works:

<Grid>
	<Popup x:Name="popup" IsOpen="True">
		<ScrollViewer>
			<controls:ItemsRepeater x:Name="SUT" />
		</ScrollViewer>
	</Popup>
</Grid>

But it’s a valid use case to not use one. I would have fix it by myself, just I’m not a huge fan of C++ 😃

[dr1rrb]

The issue is that if there is no scroller we don’t even subscribe to the EffectiveViewportChanged event. So except if you plan that a root control (like the Popup in that case) implements IScrollAnchorProvider, I think that fixing #2102 would not fix this case.