CornerRadius results in unwanted gap at the right and bottom side

[wbokkers]

Describe the bug
When I use panels with a border radius. The panels will get unwanted margins at the right and bottom.

Steps to reproduce the bug

<Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="80"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="80"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Border Background="Red" CornerRadius="20"/>
        <Border Grid.Column="1" Background="Green" CornerRadius="20"/>
        <Border Grid.Row="1" Background="Red" CornerRadius="20"/>
        <Border Grid.Row="1" Grid.Column="1" Background="Green" CornerRadius="20"/>
 </Grid>

You can see the white border between the border panels.

Expected behavior
There should be no gaps.

Screenshots

Version Info

NuGet package version:
Windows App SDK 1.0 Preview 3

Windows version	Saw the problem?
May 2020 Update (19041)	Yes

[wbokkers]

There's all sorts of things wrong with rendering when CornerRadius is set.
For example:

<Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Border Background="Green" CornerRadius="1" />
        <Border Grid.Row="1" Background="Red" />
    </Grid>

Results in:

[wbokkers]

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <Border Background="Green" CornerRadius="6,0,0,6" />
        <Border Grid.Row="1" Background="Red" />
    </Grid>

Results in:

[StephenLPeters]

@codendone and @gegao18 FYI

[codendone]

The gradient issue is tracked by this separate issue #6194. Since the cause of the gap is unrelated to the gradient, I've restored the original title of this issue to keep these separate.

[github-actions]

This issue is stale because it has been open 180 days with no activity. Remove stale label or comment or this will be closed in 5 days.