These docs are for an old release. Info on upgrading to v6
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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.