To reproduce:
1. Create and open new project
2. Open Scene window
3. Minimize Unity
4. Lock your screen
5. Put your computer to sleep
6. Wait few minutes
7. Turn computer back on and log in
8. Maximize Unity
9. Click somewhere in the Scene window with left mouse button, hold it and drag

Actual behavior: grid appears where mouse is dragged (see gif below)

Note: resize the Scene window or play the scene to remove that appeared artifacts