Did you notice the layering improvements in FileMaker 19?
Compared to older versions, the new FileMaker version now uses individual controls for layer elements. That allows the OS to do hardware acceleration and FileMaker may only need to redraw the parts which changed, leading to bigger performance.
For you, it does have the advantage that you can now better layer objects on top of each other like a button on top of a web viewer.
For next plugin we'll adjust our code to place plugin controls on the window to work better.