A gentle, bittersweet sadness.
A gentle, bittersweet sadness — yearning or desire tinged with melancholy; musingly pensive.
A soft, mixed-valence sadness — pleasurable and painful at once, which sets it apart from the whole family. Low-intensity and reflective, tied to longing and nostalgia: its trigger is typically something desired but absent, lost, or unattainable, recalled fondly rather than mourned sharply. Foregrounds the wishing/longing element that plain melancholy can lack, and the musing, pensive, almost sweet quality that heavier sadness and sorrow don't have. Bridges to the longing words like saudade and Sehnsucht.
From wistful (1610s, perhaps “closely attentive”; the “longingly pensive” sense by 1714), a blend of wishful and the obsolete wistly “intently” (which likely traces to whist “silent”).
The modern emotional sense (“longingly pensive”) is documented from 1714, postdating an uncertain 1610s “attentive” sense — a history of blending and a shift from “intent/attentive” toward “yearning, pensive melancholy.” No reliable recent-generation shift.