It seems that the content of popups do not resize since 8.1.16. Most of our popups are flex containers, with their content responsive to the size.
Since 8.1.16, the content only resizes if the popup was called with size information present. This applies to the native popup action and scripted popup called.
This is currently a known bug on our end that occurs in 8.1.16. We have an internal ticket created for this. Thank you for keeping us aware and for sharing the project.
The workaround is to set the popup’s Width/Height size to any value besides Auto.
As part of the fix for this issue we also placed automated testing which exercises the fix and verifies Views expand to fill the available space of Popups as their dimensions change during resize events. This testing is passing nightly (8.1.25Nightly), but we never saw any failures of this testing during 8.1.24 or 8.1.23. If you're encountering this issue, please provide an in-depth example of the setup with which you're encountering this issue so that we can work to replicate it and determine what our automated testing might be missing.
We specifically need to know what container type is being used for the View of your Popup. Screenshots of why you think this bug has re-emerged would also be very helpful.