FIX: ScrollViewer ignores padding when measuring children

[martinflorek]

What does the pull request do?

ScrollViewer now honors padding when measuring its children. Should fix #17158. The bug looks to be in the ScrollContentPresenter, not the ScrollViewer itself.

I am not 100% sure if this is all that is needed to fix the ScrollViewer with padding scrolling issue. This is my first look at Avalonia source code.

What is the current behavior?

Currently the ScrollViewer ignores its padding when measuring its children. And in case when it cannot scroll horizontally or vertically, it is unable to scroll to the very end, see #17158

What is the updated/expected behavior with this PR?

ScrollViewer should use its padding to limit the available size when measuring its children.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Probably none, because the current behaviour is broken and ScrollViewer cannot be scrolled to the very end.

Fixed issues

Fixes #17158
Fixes #12182

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0059920-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[martinflorek]

@cla-avalonia agree

[martinflorek]

I tested the build 12.0.999-cibuild0059920-alpha and is not fully fixed. Measuring is now good, but there is still problem with the scrolling to the very end. I will dig deeper into this and update the PR.

[martinflorek]

The problem in ScrollContentPresenter is just one part. There looks to be another problem with the ScrollViewer itself, where it incorrectly computes its Extent and Viewport properties without Padding, and may even more.

I am unable to create a ScrollViewer unit test, that will trigger all the measuring/layout code down to ScrollContentPresenter, so I will get Extent, Viewport and child size computed like in real application. Can someone help me with this please?

Another issue I started thinking about is: what does a padding in a scrolling view mean in general? Personally I want the "padding to scroll with the content", but some will probably expect a fixed padding like in other views i.e., fixed inset "frame" around the content, that scrolls within it. Maybe a scrolling view should not have a padding property at all? @timunie @trashhacker ?

[timunie]

The team will review the PR once we have enough capacity for it.

About the last question: would be interesting to evaluate what other frameworks like WPF do about the padding.