rect element with id of "container_grid_rect" fill is black in Safari within an iFrame

Hello.

I'm using this Diagram widget inside of the Claris FileMaker Pro app. This app has a 'web viewer' object that renders HTML code (and all versions, including React, Vue, etc). 
The diagram renders just fine in the desktop version of FileMaker. In its web-component, however, inside Safari, the diagram does not render correctly. 

Here's what it looks like in Chrome:

And here's what's happening inside Safari:

The Diagram widget is, in both browsers, being rendered inside an iFrame (you can see that in the DOM).

In both instances, the Diagram is using the default grid background.

But in Safari, in an iFrame, the fill of the rect element is being filled with black because it can't find the #container_pattern.

<rect id="container_grid_rect">

If I render an instance of the Diagram inside Safari not in an iFrame the grid renders just fine.

I suspect it is an issue with how the FileMaker app is rendering elements in Safari and I've reached out to their support team. But I wanted to show you as well.

Do you have any thoughts on how to resolve this iFrame issue? I can use CSS or JS to fill in this <rect> element with a color, but cannot get the grid pattern back.

What's annoying is that in my specific instance of this, I'm not using the _default grid_. I'm using a custom-built grid that is sized based on the size of the diagram.

would you have any thoughts?

Thanks

Jeremy