How to reproduce:
1. Open user-submitted project (PBSelectionProblem)
2. Make sure the ProBuilder window is open and Vertex/Edge/Face selection modes can be selected
3. Select the 'Plain Old Cube' GameObject and select Vertex, Edge or Face selection mode
4. Click on any field in the Inspector window that would cause a pop-up window to be opened(such as Material selection)
5. Move the window so that the material about to be selected is above the 'PB Object' GameObject
6. Double-click the material

Expected result: the selection does not change from 'Plain Old Cube' to 'PB Object'
Actual result: the selection changes from 'Plain Old Cube' to 'PB Object'

Reproducible with Unity versions: 2018.4.2f1, 2019.1.7f1, 2019.2.0b6, 2019.3.0a6
Could not test with Unity versions: 2017.4.29f1(ProBuilder package not available)

Reproducible with package versions: 4.0.2, 4.0.5
Not reproducible with package versions: 3.0.9

Notes:
Not reproducible on macOS
In some cases the object under the pop-up window doesn't get selected, but the cursor can be seen performing a quick drag-selection action, which deselects the first object