In Sprite Editor window Secondary Texture window flickers on selecting LINE DOWN and LINE UP buttons from the vertical scroll bar of the window. Refer attached video.

Steps to Reproduce:
1. Create a New Project with 2D Template
2. Import any Texture > Select Texture.
3. Window > 2D > Sprite editor > Secondary Textures
4. Press & hold Vertical/Horizontal scrollbar arrow button.

Actual Result:
Secondary Textures window flickers when Vertical/Horizontal scroll bar buttons are selected.

Expected Result:
The secondary textures window is scrolled up and down using a line up/down buttons gracefully.

Note:
Horizontal and Vertical scrollbar doesn't work for "Secondary Textures" in 2019.4.1f1

Reproducible on:
2020.2.0a15, 2020.2.0a7.

Working fine on:
2020.2.0a5, 2020.1.0b13

Environment:
Windows 10 and macOS 10.15