"Channel Remapping" dropdown icon not clickable under the "Mask Map" in the Inspector, refer the attached video.

Steps to repro:
1. Create a new project
2. Project > Create > Terrain Layer > Inspector
3. Mask Map > Select Mask Map
4. Click on the dropdown icon of "Channel Remapping"

Actual Result:
Channel Remapping dropdown icon is not clickable

Expected Result:
Channel Remapping dropdown icon should be click and expand

Occurring on:
2020.1.0a24, 2019.3.2f1, 2019.3.0a7

Working Fine on:
2019.3.0a6, 2019.2.13f1

Workaround: On clicking "Channel Remapping" dropdown name, the dropdown is expanded

Environment:
Windows 10 and macOS 10.14