@matimiak – Actually it doesn’t appear on every site, only sites that use dynamik JS based sizing of the grid in my experience. We use it on dozens of sites(that don’t have that type of function) with no issues, as do most of our users.
Typically this is seen when using JS sliders or similar types of elements. These sliders tend to set their width using JS rather than CSS, because of this if the document width changes (when the scrollbar gets hidden) they don’t adjust properly like they would with CSS. This causes them to be slightly narrower showing the background color behind them.
Not disagreeing that its not an annoying bug, but reality is that its the content having issues, not the popups. You can try whatever methods you like and if you find a solid solution we would be happy to include it for every user, but we haven’t found a solution that solves it for all users.
You can disable the overlays scrollbar using CSS .pum { overflow:hidden; }. Visibility hidden will not work, that will just hide the overlay itself, which is the same as my previous suggestion.
PS only had about 10 reports in the last 6 months so not as widespread as you may think considering over 50k websites.