CSS layout causes amp-iframe reload loop

Since yesterday's AMP release, all amp-iframe based code examples on amp.dev are stuck in a reload loop. Example: https://amp.dev/documentation/components/amp-accordion/. There were no changes to amp.dev which might have caused this.

Here is a minimal example to reproduce: https://forest-spark-school.glitch.me/original.html.

Update: fixed link to repro case

Start with the amp.dev amp-accordion example and the linked minimal reproduction, then compare amp-iframe behavior before and after the AMP release, focusing on the CSS layout involved. Done means amp-iframe examples no longer enter a reload loop in the reproduction and on amp.dev.

Written by the indexing model from the issue text.