[Bug] ScrollView content's padding not applied after changed in OnSizeAllocated

[Adam--]

Description

When a ScrollView contains a StackLayout (untested with other controls) and that StackLayout's padding is changed in OnSizeAllocated, the padding isn't visually shown (seems to be delayed until redrawing of the control).

Steps to Reproduce

1. Create a new Xamarin Forms solution with the blank app template
2. In MainPage.xaml use the following for it's content

<ScrollView>
        <!--  See code behind for Padding changes on rotation (OnSizeAllocated)  -->
        <StackLayout x:Name="StackLayout">
            <Label
                BackgroundColor="#DEDEDE"
                HorizontalOptions="Fill"
                HorizontalTextAlignment="Center"
                Text="Welcome to Xamarin.Forms!"
                VerticalOptions="CenterAndExpand" />
        </StackLayout>
    </ScrollView>

3. In MainPage.xaml.cs override OnSizeAllocated with

// Reference https://docs.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/layouts/device-orientation?tabs=windows
        protected override void OnSizeAllocated(double width, double height)
        {
            base.OnSizeAllocated(width, height);

            if (width != _width || height != _height)
            {
                _width = width;
                _height = height;
                if (width > height)
                {
                    StackLayout.Padding = new Thickness(60, 0);
                }
                else
                {
                    StackLayout.Padding = new Thickness(10, 0);
                }
            }
        }

4. Debug the app on Android (untested with iOS) with a device in portrait orientation and note the padding on startup for portrait orientation.
5. Rotate the device to landscape orientation and note the padding.
6. Rotate the device back to portrait orientation and not the padding.

Expected Behavior

In step 4, startup in portrait, the left/right padding is 10.
In step 5, rotate to landscape, the left/right padding is 60.
In step 6, rotate to portrait, the left/right padding is 10.

Actual Behavior

In step 4, startup in portrait, the left/right padding is 0 (Android 9) or 10 (Android 4.4) (should be 10).
In step 5, rotate to landscape, the left/right padding is 10 (should be 60).
In step 6, rotate to portrait, the left/right padding is 60 (should be 10).

Basic Information

• Version with issue: 4.3.0.908675, 4.2.0.84806
• Last known good version: Unknown
• IDE: Visual Studio Professional 16.4.1
• Platform Target Frameworks:
  • iOS: 8.0? (untested)
  • Android: 9.0
• Nuget Packages:
  • NETStandard.Library v2.0.3
  • Xamarin.Essentials v1.3.1
  • Xamarin.Forms v4.3.0.908675

Screenshots

In step 4, startup in portrait:

In step 5, rotate to landscape:

In step 6, rotate to portrait:

Reproduction Link

XFStackLayoutPaddingBug.zip

Workaround

Use Margin instead of Padding.

[PureWeen]

@adam getting the timing and propagation correct with OnSizeAllocated can be tricky. Because when you change StackLayout then OnSizeAllocated gets called a second time synchronously. When the base OnSizeAllocated is called that's what does the layout of all the children. So if you want your code to work then what seems to work is calling base.OnSizeAllocated after you have set the padding

        protected override void OnSizeAllocated(double width, double height)
        {
            if (width != _width || height != _height)
            {
                _width = width;
                _height = height;
                if (width > height)
                {
                    StackLayout.Padding = new Thickness(60, 0);
                }
                else
                {
                    StackLayout.Padding = new Thickness(10, 0);
                }
            }
            base.OnSizeAllocated(width, height);

        }

[Adam--]

@PureWeen Is it expected that Margin and Padding would have different behavior?

May be helpful to update the code in the docs at https://docs.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/layouts/device-orientation?tabs=windows#reacting-to-changes-in-orientation:

private double width = 0;
private double height = 0;

protected override void OnSizeAllocated(double width, double height)
{
    base.OnSizeAllocated(width, height); //must be called
    if (this.width != width || this.height != height)
    {
        this.width = width;
        this.height = height;
        //reconfigure layout
    }
}

[PureWeen]

My guess why the behavior is different has to do with how margin and padding are applied.

With Margin it's the parent layout being applied to the stacklayout whereas with padding it's the stacklayout applying the padding to its children

So when you change the Margin that causes the parent to recalculate but when you set the padding SL is still mid layout so the padding gets ignored

with the docs that's kind of a funny one :-/ because technically that should be fine. But, I just assumed we could out smart it by changing the order

[Adam--]

Ah good explanation Shane, thanks!