Bring control into view only if control isn't properly visible in viewport

[emmauss]

What does the pull request do?

Brings a control into view when request only if the control is currently not in the scrollviewer's viewport, or such that most of the child is visible in the viewport. Behavior was adapted from WPF's implementation. If the child is completely visible or contains the full viewport, no offset change is done.

What is the current behavior?

Requesting a child be brought into view will always align the top of the child to the top of the viewport, even if the child is fully visible. This forces the scroll offset to change reset even if the child is slightly moved into the viewport manually. This creates problems with focus, as for most focusable controls, they are brought into view on gaining focus. Focus is gained in multiple ways, most common of which is a toplevel activating after being deactivated.

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

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

Obsoletions / Deprecations

Fixed issues

Fixes #18252

[MrJul]

Minor, but this comment should read is at least partially above the viewport, or something along those lines.
The variable naming (isChildAbove) initially made me think that this was checking whether the child was entirely outside the viewport, and the comment reinforced that idea.

(Same thing for isChildBelow)

[emmauss]

Updated.

[MrJul]

This looks good, but I believe we need additional backing unit tests in this area, which are missing currently.

The following cases should be checked:

• Child is fully in view (smaller than or equal to the viewport) ⇒ It doesn't move
• Child is fully above ⇒ It's moved to the top of the viewport
• Child is partially above (intersecting with the viewport) ⇒ It's moved to the top of the viewport
• Child is fully below ⇒ It's moved to the bottom of the viewport
• Child is partially below (intersecting with the viewport) ⇒ It's moved to the bottom of the viewport
• Child is both partially above and partially below the viewport (it's too large to fit) ⇒ It doesn't move

[emmauss]

This looks good, but I believe we need additional backing unit tests in this area, which are missing currently.

The following cases should be checked:

* Child is fully in view (smaller than or equal to the viewport) ⇒ It doesn't move

* Child is fully above ⇒ It's moved to the top of the viewport

* Child is partially above (intersecting with the viewport) ⇒ It's moved to the top of the viewport

* Child is fully below ⇒ It's moved to the bottom of the viewport

* Child is partially below (intersecting with the viewport) ⇒ It's moved to the bottom of the viewport

* Child is both partially above and partially below the viewport (it's too large to fit) ⇒ It doesn't move

Added more tests

[MrJul]

LGTM!