That's an interesting suggestion. You can of course add a button at the top of your pop-up form to run a microflow to close the current pop-up and re-open it in content. Disadvantages are:
1) It's not a standard windows UI control
2) You may need to have a duplicated view without the button to show when 'In Content' or with a different button/microflow to reverse the operation
3) Not all forms that can be displayed In Content can be displayed as a pop-up
Would you be able to change/replace the action or add extra actions to the action that is kicked off clicking the maximize button in the header of a popup form? If so, would you be so kind to give me some pointers how to do that (where to start or an example)?