Cesium Window "Quick Add" buttons don't always work

Install Unity Hub 3.4.2 and Unity 2021.3.25f1.

Follow the 'Cesium for Unity Quickstart" guide,
https://cesium.com/learn/unity/unity-quickstart/

1. When creating the new project, use the "3D Sample Scene (HDRP)" template.
2. When installing Cesium from the Package Manager, use "Cesium for Unity, Version 1.2.0 - May 09, 2023"
3. Continue with the tutorial until you add "Cesium World Terrain + Bing Maps Aerial imagery".

The "+" button on the right does nothing when clicked. In fact, every "Quick Add" option behaves like this, except "Blank 3D Tiles Tileset" for some reason. The larger buttons at the top, "Add, Upload, Token, etc." seem to work ok.

Note:
- Also happens when using the "3D Sample Scene (URP)" template, but with the main Unity window resized in a different way.
- I'm using an ultra wide monitor (3440 x 1440) connected to a PC tower. Does not occur when using my laptop (1920 x 1200).
- I've been able to work around it by dragging the Cesium Window to a different place in my layout, and trying again.

Reproduce the issue in Unity 2021.3.25f1 with Cesium for Unity 1.2.0, using the 3D Sample Scene HDRP and URP templates and the Cesium Window Quick Add options. Start by comparing the non-working "+" buttons with the working "Blank 3D Tiles Tileset" and the larger Add or Token buttons, including the ultrawide layout. Done means every Quick Add option responds reliably across the described layouts.

Written by the indexing model from the issue text.