Steps to reproduce:
1) Download attached project and open in Unity
2) Open "TerrainScene"
3) Make sure that terrain brush is visible in the Scene view
4) Expand the Layers menu in the Terrain settings
5) Move your mouse above the Stroke sliders

Expected result: Terrain brush will stay in one place
Actual result: Terrain brush moves in the scene when you hovering your mouse under the Stroke sliders settings

Reproduced with:
2020.2.0b5
MacBook Pro Quad-Core Intel Core i7 2,8 GHz 16 GB ; Radeon Pro 555 2 GB Intel HD Graphics 630 1536 MB ; Mac OS X 10.15.6

Recreate with a fresh project(check attached video "Repro.mp4"):
1) Open UnityHub and create a new project
2) Create new terrain
3) In the Inspector, select brush as a Paint Texture
4) Expand Layers
5) Hover your mouse above the Stroke settings