Version 7 will be released soon.
Read the v7 docs
selectMirror
Whether to draw a “placeholder” event while the user is dragging.
Boolean, default: false
This option only applies to the TimeGrid views.
A value of true will draw a “placeholder” event while the user is dragging (similar to what Google Calendar does for its week and day views). A value of false (the default) will draw the standard highlighting over each cell.