How to reproduce:
1. Open any Scene in a new Unity project
2. In the Hierarchy window, create a Terrain GameObject and select it
3. In the Inspector window, locate the Terrain Component and navigate to "Paint Details"
4. In the "Details" section, press "Edit Details" > "Add Grass Texture"
5. In the newly-opened window, continuously click the circular button to select the Detail Texture property

Expected result: when trying to continuously open the preset window for the Detail Texture property, the preset window retains focus
Actual result: the preset window randomly loses focus when it is repeatedly opened

Reproducible with: 2020.1.0a17, 2020.1.0a18
Not reproducible with: 2017.4.36f1, 2018.4.15f1, 2019.2.18f1, 2019.3.0f4, 2020.1.0a16

Note: only reproduced with the "Add Grass Texture" window. Seems to be only reproducible with undockable windows, from which preset windows can be opened.