Reproduction steps:
1. Create a new project and make sure new Hierarchy window is enabled
2. Enable Hierarchy Integration in Project Settings (Edit > Project Settings… > UI Toolkit)
3. GameObject > UI Toolkit > UI Document
4. Select newly created UIDocument GameObject and attach Source Asset to it in the Inspector by pressing “New” button
5. Open UI Builder by double-clicking on newly created Visual Tree Asset
6. Drag and drop enough Elements in UI Builder Hierarchy to fill entire Editor Hierarchy
7. Observe Editor Hierarchy window

Actual result: Scrollview appears without a scrollbar
Expected result: Scrollview appears with a scrollbar

Reproducible with: 6000.4.0a2 (79bd4f638ac0), 6000.4.0b7, 6000.5.0a6
Not reproducible with: 6000.3.7f1, 6000.4.0a1
Couldn’t reproduce with: 6000.0.66f2 (No Enable Hierarchy Integration option)

Reproduced on: macOS Sequoia 26.2 (M4)