Entering play mode causes Sprite Editor window to split into two mirrored parts.
Image & video of this occurring included in the case comments.

Repro:
1. Start a new project.
2. Install '2D Sprite' package from Package Manager. (Window -> Package Manager)
3. Import an image. (Used a .png image to test this.)
4. Click on the image in the Project tab to open image Import Settings in the Inspector.
5. In the Import Settings:
- Set Texture Type -> Sprite (2D and UI)
- Apply Changes
6. From the Import Settings, click on the 'Sprite Editor' button.
7. With the Sprite Editor window open, enter Play Mode.

Expected Outcome: Nothing unexpected happens to any open window.
Actual Outcome: The Sprite Editor window splits into two mirrored parts.

Reproducible in 2022.1.0a5, 2022.1.0a1
Not Reproducible in 2021.2.0b9, 2020.3.17f1, 2019.4.29f1, 2018.4.36f1
Using Sprite Editor 1.0.0 - same package version is used across all unity editor versions.

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Notes:
- Tested on MacOS and Windows.
- UI Toolkit Debugger shows two "#spriteEditorWindowMainView .unity-imgui-container" components when the window splits.
- Can be returned to expected state by reopening the window, or clicking 'UI Toolkit Live Reload'.
- Window seems to still be usable, just has two mirrored views.
- Did not find other windows that did this.