How to reproduce:
1. Create a new project
2. Open the UI Builder (Window → UI Toolkit → UI Builder)
3. Dock the UI Builder tab next to the Inspector
4. Resize the left panel of the UI Builder tab (the left panel houses StyleSheets, Hierarchy and Library)
5. Observe the UI Builders left panel

Expected result: Resizing is smooth and no separation lines appear
Actual result: A black separator line appears, and the panel expands to fill the entire tab

Reproducible with: 2023.1.0a1, 6000.0.59f1, 6000.2.7f1, 6000.3.0b4, 6000.4.0a1

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Notes:

* The issue depends on the size of the UI Builder window
* When resized, you'd need to hover over the black separator in the middle of the panel to resize it again, not the border of the panel
* Depending on the size of the UI Builder window, the left panel either resized minimally, completely covers the window or in rare cases, does not resize at all
* This issue is not present in the Project tab when scaling the left panel