HERE's SOMETHING I'll bet you didn't know (I didn't). It seems Web Viewers cover all objects on a layout, but those objects are clickable if layered on top of the Web Viewer.
For example, a popover placed on top of a Web Viewer does not show on the screen, but can be activated with a well placed click, showing the area of the popover not covered by the Web Viewer layout. Same with a button: If the front most object, it is not visible but clickable, and the script runs. More interestingly, if you were trying to click a URL link that appears in the same place on the web page, it does not work–the button fires instead.
Moving the Web Viewer to the front of the layout seems to negate this behavior.
Click over and read the whole thing. And keep it in mind when developing with Web Viewers.