Main headers of some pages are hidden, for instance Diff page of the Main page of German Wikipedia 
RevisionSlider currently always display "tooltips" above the revision bar plot. On pages like  that results with contents of essentially all tooltips not being entirely visible (ie. tooltip positioned above the bars does not fit the window).
Similar problem might be observed on other diff pages (including main section header) - after scrolling down a bit with bars still available, when hovering bars tooltips are displayed above bars but partly or entirely invisible for the user.
A possible solution to the problem would be to position tooltips over the bar plot if the tooltip would fit there entirely, or show it below the bars otherwise.