The result is that users have been trained not to expect consistent UI paradigms. Every UI is hunt-and-peck. And that paves the way for this kind of exploit.
This was anticipated and partly avoided by a reasonably large modal which pops up to tell you you’re in full screen mode, and disappears after a few seconds.
Another similar exploit on desktop was to set the cursor of the page to be a very large image which would overlay the browser chrome and put some fake information there.
The issue on mobile could perhaps be reduced by having some amount of ui that doesn’t go away (safari does this in portrait mode). Another help could be to not make the ui disappear (or make it reappear) when this kind of scrolling an iframe situation arises
Even just in this case - making it look like Chrome mobile results in a different bar than Firefox mobile. If they converge more though it'd take less effort to hit more people.
Which is also why they are so abombinally large. Picking on Skype, but they are by no means the only or worst, the Android app is 71MB. There is no sane reason it needs to be that large except for all of the custom assets and custom widgets.