Created attachment 58194 [details]
It's hard to write a testcase for this, but here's what I came up with. Basically, it's possible to have a focused, but visibility:hidden element. In Gecko, that element still receives key events and allows text input. In WebKit, it receives key events, but only does one text insertion.
My opinion is that text insertion (include line-breaks) should work. If the element is receiving key events, it should also allow text insertion. I can also think of valid use-cases enabled by allowing text insertion in this case.
Related: bug 30116 and bug 40338.