When user click the link in classic UI to open the modern UI timesheet, it should open within same window. As of now it opens new page in browser. Please let us know if it's possible, we do not want to have users open classic and modern UI window open same time.
Release : 16.0.0
Component :
The link to the New UI from the Classic UI opens a new tab every time simply because this is the way external links have always worked from the Classic UI and there is no way to change it as per current design.