Hello, in terms of CSS, the way things render in front or behind is based on z-index property, so its the kind of thing that could be adjusted by style tag. either generally as a css rule in your style sheet for all elements of a certain type, or by a style tag on some shiny UI, to adjust only that.
Rather than try to push the selectize stuff forward, I have a suspicion it would be easier to move the tabpanel a little back.