This is an interesting case where security and design are in direct collision. From a security perspective you'd want the demarcation between the application itself and the untrusted content area to be as clear and obvious as possible, which would mean drawing big borders between them so thick nobody could possibly miss them. But contemporary design is all about being "clean," part of which involves making borders ra…
Taking for example the Opera 12 example I mentioned in comments above[0]: while I don't think Opera's overall UI design in 2011 was particularly visually pleasing, and certainly not as clean as Chrome's today, if you consider the UI pattern in isolation there's nothing preventing it from being done cleanly. Facebook uses the same UI pattern for the active state of its status input today.
That's just one obvious example - I'm not suggesting it's the only one. Google's Material Design guidelines advocate a lot of context-crossing - the canonical example being the "Floating action button" attaching to sheets[1]
[0] https://news.ycombinator.com/item?id=13400645
[1] https://material.io/guidelines/components/buttons-floating-a...