I've built a side panel lightbox that contains a form. The form is longer than the viewport, so the user will need to scroll the lightbox down to see the whole form. Very happy with the addition of this overflow content feature! Works fine for the user, not so much for editing.
My issues are:
Adding form fields and moving them below the visible portion of the lightbox does not seem to count as overflow content. I needed to put an invisible SVG line at a much lower point to force the lightbox to become scrollable.
In the editor, when you scroll down to edit the features on the overflowed part, the actual editor system doesn't register the scroll, and all of the edit windows are pushed up and stop aligning with the features (see image). I think this is because the editor thinks that the page under the lightbox is scrolling, and to ignore the action.
I have found a workaround for this, which is to use the Editor X zoom control (another nice feature) to zoom out until the entire lightbox is visible. Not ideal for trying to edit small elements though.