Bounding for topfill

[jmunkki]

Imagine you have two boxes stacked on each other, so that the lower box is larger and the top one is smaller. The topfill for the larger box ends up not having a boundary, so the topfill has no outline to connect to and tends to droop down and leave holes. Is there a way to detect this type of situation and add a boundary (at least a single outline) to the topfill even though it's not an outer shell?

I noticed I can alleviate this issue by using fast honeycomb infill, but that still leaves one end of the topfill with nothing to attach to.

The attachment is from a more complicated model where this also happens and the resulting print had holes near the vertical wall. A related issue is that the vertical wall that starts at the "second box" level only has the infill as a foundation, so it tends to be very weak. I guess the best solution would be to require outlines extend slightly below the surface they stand on?

[zemlin]

There are already several threads discussing this. Here's one: viewtopic.php?f=23&t=7906