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It's been suggested that it might be nice for PuTTY to support the traditional VT100 line drawing behaviour (using ISO-2022 escape sequences such as ESC ( B, ESC ) 0, SO and SI) in the same mode as also supporting UTF-8. This would allow it to simultaneously support legacy applications which ran on a VT100 and hence never output any character value above 0x7F, and modern UTF-8-aware applications.
Markus Kuhn thinks this
is wrong ("While a terminal emulator is in UTF-8 mode, any ISO
2022 escape sequences [...] are ignored"), and I (SGT) am inclined
to agree, not least because the lack of easily corruptible ISO 2022
state is a useful defence against the confusion caused by
accidentally spewing binary files into your standard output. On the
other hand, I can see the potential utility of being able to run two
classes of plausibly common application without needing a mode
luit. So I think that if we do this at all
(which isn't clear, hence priority "low"), it should be a
BJH points out that ISO 2022 requires that when we return from UTF-8 (by ESC % @) the ISO-2022 state must be the same as when we left, so if we did this then we'd have to keep a separate pseudo-ISO-2022 state inside the UTF-8 mode (and discard it on ESC % @, rather than preserving it until the next ESC % G).
SGT, 2018-05-12: a user sent a patch to implement this.