Reproduction steps:

1. Create a new project
2. Open Project Settings window, (Edit → Project Settings) and separate it into a separate tab
3. Open Build Settings window, and separate it into a separate tab
4. Drag the Project Settings tab to share a window with the Build Settings
5. Observe the result

Expected result: Two tabs are merged to share one window successfully without visual glitches
Actual result: After two tabs are merged, a white line from a window separator line appears over the text

Reproducible in: 6000.1.0a1, 6000.1.0b2
Not reproducible in: 2021.3.47f1, 2022.3.55f1, 6000.0.23f1, 6000.0.34f1

Reproducible on: Windows 10, Windows 11 (user), macOS 15.2 (M1 Pro)
Not reproducible on: No other environments tested

Notes:

* If the issue doesn’t reproduce, try to move the separator line inside Project Settings tab a bit to the right
* On mac, the issue doesn’t always reproduce