Product and version (e.g. FileMaker Pro 14.0.3)
FMPA 16.05, thought it has occurred on previous versions of 16. I don't recall this behavior in 15 but it's possible.
OS and version
macOS Sierra, 10.12.6
Mac mini (Late 2014)
Processor 2.6 GHz Intel Core i5
Memory 8 GB 1600 MHz DDR3
Startup Disk Macintosh HD
Graphics Intel Iris 1536 MB
Occassionally a window will not close. There is no script (e.g. a paused script), no dialog, no modal or card window. Other windows do close. It is only a single window that doesn't close. In the Quicktime example (see attached), the window in question was the second of three windows opened. All windows were opened manually (Shift-command-L).
The window does not freeze the entire platform. E.g. I can still access the menus such as File, Edit, View, etc. And I can still open the layout manager and open other windows. I can also navigate the "stuck" window to other layouts. Once in another layout, it will still not close.
How to replicate
It occurs randomly, in different solutions.
Workaround (if any)
Attached is a Quicktime Movie (no sound).