HeightRequest is not honored if content is added below - xamarin

Question

I am trying to lock the height of a header - or basically any kind of content on the top of a page. However, once scrollable content is added below the height changes. A simple example:
<ContentPage.Content>
<StackLayout Padding="0" Margin="0" Spacing="0">
<Frame Margin="0" HeightRequest="150" BackgroundColor="red" CornerRadius="0" HasShadow="False" Padding="0"></Frame>
</StackLayout>
</ContentPage.Content>
This works fine. However, adding a lot of simple label lines in a scrollview somehow changes the height of the frame.
<ContentPage.Content>
<StackLayout Padding="0" Margin="0" Spacing="0">
<Frame Margin="0" HeightRequest="150" BackgroundColor="red" CornerRadius="0" HasShadow="False" Padding="0"></Frame>
<ScrollView>
<StackLayout>
<Label Text="aaa" /> <!-- Repeated 54 times however removed here for simplicity-->
</StackLayout>
</ScrollView>
</StackLayout>
</ContentPage.Content>
The results end up like this:
Why does it happen and can I do anything to avoid it? And why is the height reduced to exactly that height? Thanks!

Answer 1

It doesn't seem like you're limiting the height of the ScrollView. So considering you're merely doing a HeightRequest, it may change as needed to fit everything.
It's usually safer to use Grids. In this example, something like this could help you:
<ContentPage.Content>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="150" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<Frame Grid.Row="0" Margin="0" BackgroundColor="red" CornerRadius="0" HasShadow="False" Padding="0"></Frame>
<ScrollView Grid.Row="1">
<StackLayout>
<Label Text="aaa" /> <!-- Repeated 54 times however removed here for simplicity-->
</StackLayout>
</ScrollView>
</Grid>
</ContentPage.Content>