zotero/scss/elements/_contextPane.scss

context-pane {
    -moz-box-orient: vertical;

    #zotero-context-pane-item-deck,
    #zotero-context-pane-notes-deck {
        position: absolute;
        height: 100%;
        width: 100%;
    }

    .zotero-context-notes-list {
        min-width: 0;
        min-height: 0;
    }
}