JetBrains: Fix the JCEF flickering at first load

Created by: vdavid

Problem

When the JCEF component in the modal first loads, its background is white at first, then changes to grey as the page loads. 10-sec Loom demo This looks weird and we don't want this.

Solution

The white background might come from two places: a.) the page background b.) the JCEF component background

If it's a.) then the proposed solution is to dynamically set the body background early. If it's b.) then the proposed solution is to render a different Swing component over the JCEF until the document loads.