The screenshot shows my library after dragging a pdf. Selection works as normal – any reference I selected highlights in blue, but the phantom grey selection, which was the reference selected before adding the pdf, remains grey. This has happened a couple of times previously but I've only thought to report it now. Zotero 10.0.5 on a Macbook running 14.8.9
https://s3.amazonaws.com/zotero.org/images/forums/u6613072/679sbkvh1ovfrx9eh8h1.png