It seems a popover will always expand into view from its top left corner, rather than from the position of the arrow connecting it to its button. This breaks visual continuity between the user's action and the information coming into view.
1. Add a popover button to a layout and note the orientation of the popover relative to the button.
2. In Browse mode, click on the popover button.
The popover should appear to expand from a point on the edge of the button corresponding to the popover's orientation.
The popover appears to expand from what would be the top left corner of the expanded popover.
I'm using FileMaker Pro Advanced 126.96.36.1992 on macOS 10.13.1.