Jbrowse JBrowse

Fixed bug where ghost for annotations being dragged would sometimes disappear behind other tracks, when other tracks have a background color set (discovered when working on MAKER dark background styles).
Still need to clean up:
base zIndex on retrieving actual zIndex from CSS rather than hardwiring check on render relative to sequence track -- could make ghost disappear if over seq track. But I think rendering over seq track may actually be desirable behavior to essentially get DNA overlay for an evidence element. https://github.com/GMOD/jbrowse/commit/bf5bced6c3807936646ba2abe8cf...