Describe the bug

A clear and concise description of what the bug is.

When creating a TokenizingTextBox with the Header property set, the header is displayed is inside the input area and not on top of it.

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

• Can this be reproduced in the Sample App? (Either in a sample as-is or with new XAML pasted in the editor.) If so, please provide custom XAML or steps to reproduce. If not, let us know why it can't be reproduced (e.g. more complex setup, environment, dependencies, etc...)

Steps to reproduce the behavior:

1. Create TokenizingTextBox with Header

Expected behavior

A clear and concise description of what you expected to happen.

Header should render just like other text input controls, i.e. on top of it and not inside the input area.

Screenshots

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XAML:

<TokenizingTextBox Header="Some text"/>

Environment

NuGet Package(s):

Package Version(s):

Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [ ] May 2020 Update (19041)
- [x] Insider Build (build number: 19042)

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [ ] May 2020 Update (19041)
- [ ] Insider Build (xxxxx)

Device form factor:
- [x] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio
- [ ] 2017 (version: )
- [x] 2019 (version: )
- [ ] 2019 Preview (version: )
- [x] 2022 Preview

Additional context

Add any other context about the problem here.