When moving, dragging and resizing the menus in Unity 2018.3.0b9, particularly the game menu, Unity is laggy, unresponsive and can drop the selection.

Sets to reproduce
- Open Unity 2018.3.0b9
- You can create a new project or use the project files from the video located here (https://tinyurl.com/y7dkgbcc)
- Resizing the game window moving it over the scene menu.
- Notice issue >> Lots of lag, selection drops, and red flashes appear in the menus. See video.

Broken: Unity 2018.3.0b6, 2018.3.0b9, 2018.3.0b1
Working: Unity 2018.2.1f1. 2018.3.0a11

macOS: Mojave 10.14
Graphics: Radeon Pro 560