Steps to reproduce:

1. Create a new project
2. Go to "File" ->"Build Profiles" 
3. Resize the "Build Profiles" window (make it much bigger)
4. Also, resize the "Platforms" section (make it wider)
5. Dock the "Build Profiles" window next to the "Scene"
6. Observe the "Build Profiles" window.

Actual results: Instead of one separation line, a user sees two. A few screenshots and a video are added below in the "Attachments" section.

Expected results: A user should always see one line.

Reproducible with versions: 6000.0.11f1, 6000.0.14f1.

Not reproducible with versions: 6000.0.10f1

Can’t test with version: 6000.0.8f1 because the line is stable in this version.

Tested on (OS): Sonoma 14.6

Culprit PR: https://github.cds.internal.unity3d.com/unity/unity/pull/46899
On this line:
 https://github.cds.internal.unity3d.com/unity/unity/blob/f08bf0a1a945c6ab8972030f7231b2ad4403e5bb/Modules/UIElements/Core/Controls/TwoPaneSplitView.cs#L466

The m_FixedPaneInitialDimension is not updated after the pane is resized, so the dragline is misaligned after resize. Reverting to the old code that uses dimension would fix this, but I am not aware of other caveats in this, so I am assigning this back to the original author.