PopupOpacity when opening a dialogform

I'm opening a dialog form using:
The background is greyed as expected.
On my page, I have a TWebResponsiveGridPanel. If the grid is scrolled, the greyed background is no more working correctly. The greyed part is only on the original screen and not scrolled.
Page not scrolled:

Page scrolled:

Do you have a solution for this problem ?

I'm using Delphi 12 and my WebCore version is

This is not trivial to automatically handle in the layer with opacity that is put on top of the page before the dialog is shown.

A possible solution meanwhile is to block the scrolling when the dialog is shown and unblock again when the dialog closes.
A possible technique for this is described here:

Thanks for the tip, but this not directly my problem. My problem can be seen on the second picture. Once the grid scrolled, I click on a field calling the popup dialog. You can see the popup dialog in the middle of the screen. This is correct, but the greyed part in not centered on the screen. This is my problem.