Summary
I suggest considering adding a Separator element to the Window Size Menu Items list to separate resolution sequences.
Problem / Use Case
I use four resolution groups:
- Resolutions that exactly match my monitor's aspect ratio (WQHD / UW6K / 43∶18).
- Resolutions that are very close to it.
- The most popular resolutions according to Google Analytics and W3Counter (for website testing)
- The most popular resolutions according to Steam (for game testing)
Current Workaround
Title: —————
Left: N/A
Top: N/A
Width: 1
Height: 1

Proposed Solution
A button that adds a horizontal line, unselectable even on hover, is sufficient. The height is less than the height of clickable list items.
UX & Usability