I noticed this when testing the ripple effect of the paper-buttons. Double-clicking the button results in the inner label text getting selected.

screenshot:

This is quite ugly, one way to prevent this from happening is adding some css to the inner label:
All of the correct CSS variations are:

.noselect {
    -webkit-touch-callout: none;
    -webkit-user-select: none;
    -khtml-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
}

Buttons is one place where this is happening, This can be reproduced on your github.io page as well. Toolbar buttons also have this problem.