Sevral applications in our products have the "Show Fonts" in the font menu to show the font panel to select font used in the table. However the menu doesn't work in macOS 10.12.
It is caused by changes of the behavior of the show font API, and we found an alternative way. But it is not implemented yet because of an another issue.
The table in the window shows the strings by the system font, ".AppleSystemUIFont" as default. The system font is changed in macOS 10.12 and it is San Francisco Font Family.
However it is not a sole font, but several fonts in the family are used base on size and weight. And the problem is that the font is not shown in the font panel. So you can not select the San Francisco once you change the font.
This is the reason why we don't fix the issue at this moment. However you can change the size via the menu "Bigger" and "Smaller" in the font menu.
Please let us know if you have any solutions or suggestions. We will appreciate it.