How to get rid of darkness in Swing-JavaFX (JFXPanel)?

Question

Java

When using JFXPanel, I noticed such a not very error, not a bug, but most likely a misunderstanding of the operation of this component. The problem is that the background of my JFXFrames (Jframe + JFXPanel) when using my favorite transparency from FX turns out like this.

Explanation:
.root is the main window. Let's make it white and translucent.
The program is shown below on a white background.

(according to my logic, since I made my panel have a white background, then it should not be visible on a white background)
If you do rgb (255, 255, 255) there is no "gray".

It also looks good in white rounded windows

. I think I missed something somewhere.
Code
Prompt.