I've been having this problem for about three or four months now, which may have been after an update of some kind. Sometimes when I drag a monster out of the Compendium tab, the token does not get placed on the map and instead a highlightable (but useless) field appears in the upper-left corner of the Roll20 screen, obscuring the first two panels on the control palette. I can't drag any tokens out after that unless I refresh the entire browser. This is frustrating in the middle of combat (such as when my druid wants to transform into a new beast), as I have to refresh my browser and open all other stat block windows that I had open before. I can't figure out the rhyme or reason behind this, except that perhaps it has to do with having creature stat block windows open when I drag the token out. Below is a depiction of the glitch after attempting to drag a gnoll fang of Yeenoghu onto the board.