ScrollVisibility js error with IE9, possibly other browsers
With github wt (3.3.7-rc1-2-g4606fda), the use of setScrollVisibilityEnabled(true) can result in a JS error with IE 9 and possibly other browsers that do not support MutationObserver. This can be observed with IE 9 by scrolling down through the scrollvisibility feature example. The exception_description is "Could not complete the operation due to error 80020101." This appears to be caused by line 146 in ScrollVisibility.js: "mutObserver.disconnect()".
Attached, for your review, is a lightly tested patch that seems to help with this issue.
NOTE: The issue is also likely to be present in wt4, as the same ScrollVisibility.js is used.