When using the Set Web Viewer script step in WebDirect the web viewer location is reset when resizing the browser window.
It is also reset if the layout is edited.
The workaround is obviously to use a Global Field (or possibly a global variable) as the source of the web viewer, however I don't think this is by design and so is a bug. In FileMaker Client it works as expected.
Tested in WebDirect 14, though I think it exists in 15 too.