How to reproduce:
1. Open the attached project "PrefabBugRepro.zip"
2. Focus away from the Unity Editor by left-clicking another program or the desktop
3. Left-click anywhere on the Unity Editor

Expected result: the Unity Editor regains focus, nothing else happens
Actual result: speedtree assets are reimported after the Unity Editor regains focus

Reproducible with: 2019.1.0a15, 2019.1.6f1, 2019.2.0a1, 2019.2.0a6
Not reproducible with: 2019.1.0a14, 2019.2.0a7, 2019.2.0b5, 2019.3.0a1, 2019.3.0a5
Could not test with: 2018.4.2f1 (missing packages)

Notes:
-steps 2-3 can be repeated indefinitely with the same result
-the Unity editor cannot be exited out normally as clicking anywhere on the window header (including the exit button) will always trigger the mentioned reimport
-switching focus with the keyboard reproduces the issue as well