Fixes TextBox measure logic for MaxLines scenario

[billhenn]

What does the pull request do?

The current TextBox control templates apply the TextBox.Padding (like here) to controls outside of the ScrollViewer. But that leads to margin around the scrollbars, which might not be a desired appearance.

This code update walks up the visual tree between TextPresenter and ScrollViewer to look for additional margin/padding that doesn't appear in the Simple or Fluent themes but might in custom control templates. It appends the vertical space so that MaxLines measures correctly in those cases. For instance, the same kind of TextBox with Padding applied within the ScrollViewer would then render like this:

What is the current behavior?

Current behavior does not factor in the any Margin or Padding settings that may have been set in a TextBox control template between the ScrollViewer and TextPresenter. This scenario doesn't happen in Simple/Fluent themes but custom themes may run into measure issues.

Without the PR's code update, the second scenario measure incorrectly like:

What is the updated/expected behavior with this PR?

The update allows for scrollbars to align right next to the outer border in custom themes, while still having padding within the ScrollViewer as seen above.

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

Explained above.

Checklist

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

Breaking changes

None, the Simple/Fluent themes will still behave the same since there is no Padding set in the scenario that this update looks for.

Obsoletions / Deprecations

None

Fixed issues

None

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0038562-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]