We ran into this issue before when using the release prior to the April 2018 release, but this blog post gave a workaround which resolved the issue.
https://community.qlik.com/blogs/qlikviewdesignblog/2017/06/08/chart-tooltips-and-scrolling-in-a-mas...
After upgrading to the April release, the issue has returned. Does anyone have any idea how to fix this?