How to reproduce:
1. Open the attached project (1252332.zip)
2. In the Project window select the "Sprite123" sprite
3. In the Inspector window press the "Sprite Editor" button
4. In the top left of the Sprite Editor window press the "Slice" button
5. Open the " Type" dropdown (notice how the Slice panel closes)
6. Select "Grid By Cell SIze" option in the dropdown
7. In the top left of the Sprite Editor window press the "Slice" button again
8. Notice the "Type" dropdown's value

Expected result: The Slice panel doesn't close when a dropdown is opened and the value of the "Type" dropdown is set to "Grid By Cell SIze"
Actual result: The Slice panel closes when a dropdown is opened and the value of the "Type" dropdown is set to "Automatic"

Reproducible with: 2019.3.15f1, 2019.4.0f1
Not reproducible with: 2019.3.14f1, 2020.1.0b12, 2020.2.0a14

Notes:
1. Tested on Windows 10 and Ubuntu 18.04 (doesn't reproduce on Windows 10)
2. The same behavior occurs with all of the Slice panel's dropdown elements
3. Also reproducible in 2018.4.23f1, but not listed to keep the reproducible versions easy to understand. More information about which versions this issue used to reproduce on is avalible in the old case