scrolling/resizing 24 bit ANSI/VT100 image render is pathologically slow; Unicode graphic drawing characters mis-rendered
iTerm2 3.1.5beta1, MacOS 10.13
When using https://git.gnome.org/browse/vte/plain/perf/img.sh (requires ImageMagick) to render a JPEG in iTerm2, performance is reasonable (though slower than XTerm, which is not particularly fast) ... until the screen scrolls. An 80x24 window is already unusably slow, and anything taller/wider becomes pathologically slow, with beachballs and unresponsive windows. Activity Monitor shows CPU usage pegged at 100%. Resizing windows shows similarly slow performance, suggesting the problem is unrelated to terminal input processing.
This profile was taken scrolling this image in a larger window, I think 157x66. Sample_of_iTerm2.txt
This debug.log was taken with a 208x65 window, waiting a couple of minutes for the image to finish scrolling. debuglog.txt.xz
There's also an issue with rendering of Unicode graphics characters in iTerm2, see attached screenshots of iTerm2 and XTerm. These are the same picture of the Star Trek Enterprise bridge viewscreen. XTerm gets the 24 bit colors wrong (apparently it reduces them to a 512-color palette) but the drawing right, iTerm2 is the reverse.
Possibly related to #6241 (closed). Neither of these problems is an actual problem or significant use case for me, I discovered these while working on 24-bit support for Mosh.