Since the last version or two ShowModal with a popup isn't working correctly as it is not obeying the size of the form. It was working ok before so something must have changed. The height goes off the screen vertically.
Further to this. Your example works. But if a simple example is created from scratch it exhibits the same problems I am encountering in a larger app. Please see http://test.simplyaccounts.net/Popup.zip
Can you please investigate why this is happening. I am using Delphi 10.3.1 and Firefox 67.0.4.
To solve this for now, give ElementClassName a value (don't leave it blank) for the form that contains the webpanel1 (Form1).
The javascript creating the form looks at this, and if it is blank set's height to 100%, so if you give it a name, it won't specify the height and it won't scroll off the screen.