How to position input z-index in md tabs?

Question

css

Good day, friends!
Using the Angular-intro script, in md-tabs, there is a problem that the input is highlighted, but for some reason its z-index is less than necessary, thus it is not possible to select it with the mouse. What's the matter?
PS If you use it without md-tabs, everything is fine.
I am attaching a visual example: codepen.io/zircon163/pen/vGpyMO
Thanks a lot in advance!