Not really a bug, more of a side effect for how layout rendering works in WebDirect.
Essentially what happens is if the user resizes the window, the server will "redraw" the layout based on the new window specs and send the newly rendered layout to the client. This essentially causes a "refresh" that is resulting in the action that you see.
Do you have any anchoring/resizing on the web viewer object that would for-sure be affected by this?
If it's not a bug, and by design, then it should be documented in the web viewer guide or the Set Web Viewer documentation. It isn't and therefore it looks like a bug - especially as FileMaker Pro operates differently.
I can reproduce it on FMS15 (without script). It occurs on simple static URL in web viewer, user interaction is reset by resizing. It is not browser default action for iframe, so WEBD or underlying VAADIN do it.
I prefer report it as bug if you want, or at least you can make "product idea" for it.
A webviewer is not an iframe. It does not (and is not expected to) behave like a full on browser when rendering website content.
The fact that you are able to reproduce it would make it less of a bug.
Submit a product idea, not a bug report, and you may get more traction.
Is not but using it on WebD. (I don't know about FMS16 or later.)
I agree WebD is not full feature client like FMPro, but FMI is selling them as "seamless" now, so the difference is expected minimum for user, at least it should be documented.
Saying about me, this issue is not so serious since I rarely resize browser window