handler = e => {
const i = Number(e.target.parentNode.id)
const m = btnSize(e.target)
if ( minSizeAt(i) < m ) { return }
const i1 = nextIndex(i)
const i2 = nextIndex(i1)
const m1 = minSizeAt(i1)
const m2 = minSizeAt(i2)
if (m<m1) {
doc.children[i1].prepend(e.target)
return
}
if (m<m2) {
doc.children[i2].prepend(e.target)
return
}
}