Thank you for your post.
I can replicate this issue only if the Show scroll bars setting (System Preferences -> General) is set to "When scrolling". This issue has been previously reported, and our Development and Testing departments are aware of this issue. Another customer on the FileMaker forum has also encountered this issue:
I have attached your post to the original report.
As a workaround, change the Show scroll bars setting to either "Always" or "Automatically based on mouse or trackpad".
"Automatically based on mouse or trackpad" was the setting I was originally on and with which I first experienced the issue. Switching to "Always" as a workaround seems to have fixed the problem.
Thank you very much.