You may try to place popover header or footer part.
I agree that it's annoying and AFAIK there's not much you can do about it, except redesign your interface.
Instead of a list layout, have a (self) portal occupying 2/3 of the screen on the left, while 1/3 on the right is the place for a related records portal based upon the element selected on the left,
Open a modal window instead of a popover, hosting your related records portal
or (depending on the related data)
have a tooltip doing an SQL and displaying relevant info from the related records.
I don't like this too - when using the new Inspector 5, if one moves a finger on the magic mouse..
This is v1 or v0 if one prefers. Any new feature has bugs and things to sort out and the fact that a popover should create a modal window and take the full focus has yet to be implemented. If the cursor is outside the popover window, whatever it is over can receive the focus such as the scroll button on my mouse causing the list to scroll beneath the popover.
IMO or IMHO the popover should become modal and developers required to put a close me button on the popover or an action button that contains the Close Popover command within its steps. Or the popover should close instantly if the list begins to scroll. Popovers should act like menu item selections. If the cursor leaves the area, either the popover is modal and the location of the cursor has no affect or the popover closes like a menu item will close itself. The current method is sloppy and unfinished.
It seems to me that this is probably OS behaviour rather than FM—I have experienced similar behaviour in other applications. Nevertheless, it ought to be possible to design your FM layout to prevent it, as suggested already by siplus.