@ianhitt I found the issue.... The Title tags in the "New Posts" and other third-party add-ons had "white-space: nowrap;" but also "overflow: hidden;". WHILE Safari 9.x did ignore that in the past it now makes this a problem in Safari 10. But I wonder why this only appeared with the last UI.X update... I havent changed anything in my sub style and let me repeat it here: the issue is in the parent UI.X style too, so I recognized it as an UI.X issue and not a sub-style issue. But removing the "nowrap" solved the issue.
EDIT: It somehow needs a fix for the "flexbox" function though... because if someone really needs to use the nowrap css style the flexbox will extend. Maybe the CSS for responsive mode needs to be adjusted...
EDIT #2: Look at "flex-basis: 280px;"... that seems to be a problem under specific circumstances. The widget extends over more than the flex-basis if the sub elements of the widget have a "white-space: nowrap;" included... Please look into this.