Fix Sticky ScrollHeader causing unwanted scrolling with Popups on ListItems

[Difegue]

Fixes #3415

I took a quick look at the ScrollHeader bug and it was simple enough so:

The Scrollheader's StickyHeaderBehavior adjusts scrolling when focusing an element of the ScrollViewer if it's slightly hidden by the header. This imitates standard ScrollViewer behavior.
However, ScrollViewer.GotFocus also triggers if we open a Flyout on a ListItem, but since the scrolling calculation didn't plan for that, it ends up scrolling the list by the height of the Flyout.

An easy fix for this is to check if the focused element is really part of the list and not just a popup, which can be done by checking if the element has a parent or not.

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

If a Popup/Flyout of any kind is assigned to either the ListView or its items, opening it by right-click will scroll the ListView by the height of the Flyout if a Sticky ScrollHeader is used.

The bug can be easily repro'd in the sample app's ScrollHeader page by adding a Flyout to its ListView:

<ListView.ContextFlyout>
         <Flyout>
            <Button Content="Hello!" />
        </Flyout>
</ListView.ContextFlyout>

What is the new behavior?

Showing the Flyout no longer scrolls the ListView.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

[ghost]

Thanks Difegue for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[Kyaa-dost]

Thanks, @Difegue!! Looks good to me 🚀

[ghost]

Hello @michael-hawker!

Because this pull request has the auto merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

p.s. you can customize the way I help with merging this pull request, such as holding this pull request until a specific person approves. Simply @mention me (@msftbot) and give me an instruction to get started! Learn more here.