Hello. There is nothing to worry about. I think you just misunderstood and this is most likely not a bug, just want to clear that up.
But you are free to report here : github.com/Scirra/Construct-3-bugs/issues
No one will stop you.
Let me explain, in your case, it looks like the Preview Window is unfocused because of your Virtual Keyboard is interfering. This is not a bug. Instead, it is how your applications interact with your Virtual Keyboard.
About Virtual Keyboards, they work in different ways on various devices. In my current tests, just to make sure, I found out that my Virtual Keyboard doesn't unfocus the Preview but it doesn't allow controlling the 8-Direction behavior using the W,A,S,D but allows the arrow keys (which I only have a "left" and "right"), this is because, my Virtual Keyboard on my Gaming Laptop is a texting-purpose keyboard.
The test was done on both Construct 2 and Construct 3 with both C2Runtime and C3Runtime.
As I said, this is a misunderstanding on how Applications interact with Virtual Keyboards. And this is most likely not a bug.
I hope that explains that. Have a nice day and happy game development!
And Merry Christmas too.