Steps to reproduce:
1. Create and open a new project
2. Open Edit > Project Settings > UI Toolkit and enable Enable StyleSheet Editing Mode
3. In the Project window, create a Style Sheet (Create > UI Toolkit > Style Sheet (USS))
4. Double-click the .uss asset to open it in the UI Builder
5. Look at the Hierarchy pane, and at the UXML Preview pane at the bottom of the viewport
Actual results: The Hierarchy root row and UXML Preview pane header show <unsaved file>.uxml instead of the asset being edited, while the canvas title and StyleSheets pane correctly show Simple.uss two contradictory document identities in the same window.
Expected results: The Hierarchy root row and UXML Preview pane header show the preview document's name or a neutral "Preview" label, consistent with the canvas title and StyleSheets pane.
Reproducible with versions: 6000.7.0b1, 7000.0.0a1
Can’t test with versions: 6000.6.2f1 and lower (Feature not available)
Tested on (OS): macOS 26.6.1 (Apple silicon)