Product and version FileMaker Pro 15.0.1
OS and version Mac OS X 10.10
Hardware iMac using a Magic Mouse
When the content of a popover cannot be displayed fully it will scroll. Vertical scrolling works fine as you can scroll up and down. When you reach the bottom, the event is passed to the object underneath, the layout for instance. In FileMaker 14 you could also scroll horizontally if the content of your popover was wider than the displayable area (window). Although the behaviour was rather odd as both the popover content and the layout underneath were moving it was at least possible to move the popover content right and left. In FileMaker 15 this stopped working completely.
The attached file opens a layout with a popover, both bigger than the initial window size.