Invert meaning of DECSDM (private mode 80)
There has been some confusion whether enabling
DECSDM (private mode 80) enables or disables sixel scrolling.
Foot currently enables scrolling when
DECSDM is set.
The confusion is apparently due to a documentation error in the VT340 manual, as described in https://github.com/dankamongmen/notcurses/issues/1782#issuecomment-863603641. And that makes sense, in a way: the
DECSDM stands for Sixel Display Mode. I.e. it stands to reason that enabling that disables scrolling.
Anyway, this lead to https://github.com/hackerb9/lsix/issues/41, where it was eventually proven (by testing on a real VT340), that foot, and a large number of other terminals (including XTerm) has it wrong: https://github.com/hackerb9/lsix/issues/41#issuecomment-873269599.
It looks like at least XTerm will change its implementation. I don't see why foot shouldn't follow suite.
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