Product and version: FileMaker Pro Advanced 17.0.4
OS and version: macOS Sierra, 10.12.6
Hardware: MacBook Pro
On a Mac, there is an option in the Trackpad settings for "Tap to click." If you use that feature, a tap on the trackpad performs both a "mouse down" and a "mouse up" event - aka a "click." This is a useful feature for multiple reasons, including as an accessibility feature when physically clicking a trackpad will not work easily.
However, it seems that FileMaker's "pressed" state for a buttonbar looks at the mouse-down/mouse-up pair separately, with no minimum time in between those two events. So, it seems as though the quick mouse-down/mouse-up sent by "Tap to click" results in a "pressed" state that is so short it is usually not visible to the user, making it hard to receive feedback that you successfully clicked the button.
Instead, when looking at the mouse-down-mouse-up event pair to determine "pressed" state, FileMaker should have a minimum time duration for "pressed" so that the user can actually see it happen. Even a tenth of a second would be more than enough time to let a user know the button was (briefly) pressed. As it is, anyone using that accessibility feature has a hard time knowing that they managed to click the button.
How to replicate
Turn on "Tap to click" in the Trackpad settings in macOS.
Click a button bar segment and notice that the "pressed" style appears extremely briefly, usually so quickly that you cannot see it.
Attached is a simple demo file where you can see the number of times a click happens.
Workaround (if any): Unclear if there is any across-the-board workaround. You can have the script run BY the button bar segment update some other interface element (as in the demo file). This is not always a reasonable workaround option.