Asset Selector window disappears when viewing tooltip in ProBuilder window

Steps to repro:
1. Create a new or open attached project.
2. Install "ProBuilder" package from Window > Package Manager.
3. Tools > ProBuilder > ProBuilder Window
4. Main Camera> Background
5. Open color picker window
6. Hover mouse pointer on any tool in "ProBuilder" window to view its tooltip.

Actual Result: Color picker window disappears

Expected Result: Color picker window should not disappear

Note:
1. It occurs for all pop-up windows like Sprite selector or material selector window.
2. On mac tooltip information appear on top of Asset Selector window if it's positioned above Pro Builder window.

Environment:
Windows and Mac

Occurring with:
2019.1.0a6, 2018.3.0b8, 2018.3.0b6

Working fine:
2018.3.0b5, 2018.2.0f2

Probuilder package version: 3.0.9