🔸.scrollBox
browser ⟩ DOM ⟩ type ⟩ Element ⟩ boxes ⟩ Element+boxes ⟩ .scrollBox
( .scrollWidth, .scrollHeight ) = .paddingBox + overflow
.paddingBox：.clientWidth/.clientHeight = content + padding
.scrollBox ：.scrollWidth/.scrollHeight = content + padding + overflow
if no overflow, they are the same.
( .scrollWidth, .scrollHeight ) will round to integer.
use elem.getBoundingClientRect(), if fractional value is needed.
element .paddingBox
document document size
viewport size.
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