Reproduction steps:
1. Create a new Unity Project
2. Open Collaborate window: Window -> Collaborate
3. Click "Set Project ID"
4. Create Unity Project ID: Edit -> Project Settings -> Services -> Collaborate
5. Enable the Collaborate Service
6. Drag the mouse on the Collaborate window, search bar's bottom edge

Expected result: The Collaborate window search box doesn't flicker
Actual result: The Collaborate window search box flickers

Reproducible with: 1.3.9, 1.5.7, 2.1.0-preview.6 (2020.3.1f1, 2020.3.8f1)
Not reproducible with: 1.3.9, 1.5.7, 2.1.0-preview.6 (2020.3.0f1, 2021.1.0a1, 2021.1.8f1, 2021.2.0a17)
Can't test with: 1.2.15 (2018.4.35f1), 1.2.16 (2019.4.26f1) (No search box)

Note:
- Reproducible only on the light Unity editor's theme
- The issue depends on Unity editor's version, the Collaborate package version has no effect