Site editor: fix padding and margin visualizer positioning

[talldan]

What?

Fixes #41823

TLDR - Ensures frameOffset is not applied for popovers that render inline in the site editor's iframe.

Longer explanation:
The site editor has its content within an iframe, and usually most popovers (like the block toolbar) render in the main document. An x/y offset needs to be applied to these popovers so that they're in the correct position on the screen.

There are two situations where this offset doesn't need to be applied. If the popover and its anchor/reference are both not in the iframe (for example, a popover for the block inspector). Or if the popover and its anchor/reference are both in the iframe. The latter is the situation for the padding margin visualizers—they need to be rendered in the site editor's iframe so that they have access to the padding/margin CSS variables for content. But they were still incorrectly receiving the iframe offset.

How?

Adds an extra check to see if the popover and its reference/anchor element are in the same document. This required gaining access to the floating element's owner document, which adds a little bit of extra code.

Testing Instructions

1. Open the site editor
2. Insert a cover block
3. Adjust padding / margin
4. The visualizers should match the block position

Screenshots or screencast

Before

After

[jasmussen]

Excellent, thank you for working on this! Here's a GIF showing the padding visualization looking correctly in both the post editor and the site editor:

Code looks good, but would appreciate a sanity check. It seems like this did not have anything to do with the root padding property after all!

[ntsekouras]

Thanks Dan! 💯

[ciampo]

Thank you Dan 🙏