Selector gets stuck in UI if scroll is used while dragging

Steps to reproduce:

1. Create a new URP project.
2. Assets > Create > UI Toolkit > UI Document.
3. Open this in UI Builder.
4. Import test.uss attached to this ticket to the project.
5. Under StyleSheets > + > Add Existing USS - pick the imported test.uss.
6. Resize the StyleSheets section so it is short enough to scroll if it isn't already.
7. Click the selector at the bottom of the list, and drag while scrolling up.
   • If the selector doesn't get stuck, repeat this step and then scroll down without letting go of the mouse.

Actual results: The selector gets 'stuck' and the UI remains floating over the stylesheets area, even when you let go of the mouse. It still reacts to being hovered over, but can't be moved. It will not go away until you click on the selector in the list again.

Expected results: No UI issues.

Reproducible with versions: 6000.1.0b3, 6000.0.35f1, 2022.3.55f1, 2022.2.0a17

Tested on (OS): Mac OS Sequoia 15.2, Windows 10

Notes:

• I have run into this many times while using UI Builder over the last year or two but could not find a repro for it until now.
• Alternative to test.uss is to make a USS with enough selectors to activate the scroll bar and allow you to scroll a little.
• Also occurs if you have multiple USS files and are trying to drag a selector from one to another.