Clicking on dropdown icon of Padding property for Rect Mask 2D doesn't expand however it expands when clicked on padding text
Please refer to the attached video.

Steps to repro:
1. Create a new project
2. Hierarchy >right-click > Create Empty
3. Inspector > Add Component > Rect Mask 2D
4. Click > Padding expand icon

Actual Result:
"Padding" property dropdown icon is unresponsive OnClick.

Expected Result:
"Padding" property dropdown icon should respond and open.

Occurring on:
2020.1.0a18, 2020.1.0a15

Note :
Soft masking for RectMask2D as well as a masking offset is introduced in 2020.1.0a15.

Environment:
Windows and Mac