Now I see (I don't know how I missed it) that there is an altitude slider at the very bottom of the selection panel that one gets if one opens the overlay selection (i.e., clicking on "Wind") in the upper right -- so I suppose I can just avoid adding this slider to the main window.

(It might still be nice to know why the above doesn't work, and if there is any way to create and open a slider on the main window. Note that Javascript console information doesn't seem to help for that -- there were no related error messages there.)