The ANSI escape codes \e[0A (cursor up 0 lines) and \e[0B (cursor down 0 lines)
are not well defined and are treated differently by different terminals. In
particular xterm treats 0 as a missing parameter and therefore defaults to 1,
whereas rxvt-unicode treats these escapes as a request to move 0 lines.
However the use of these codes is unnecessary and were really just hiding the
fact that we were not correctly computing diff when adding a new line. Having
added the new line to the ids map and output the corresponding \n we need to
then calculate a correct diff of 1 rather than leaving it as the default 0
(which xterm then interprets as 1). The fix is to pull the diff calculation out
of the else case and to always do it.
With this in place we can then avoid outputting escapes for moving 0 lines.
Actually diff should never be 0 to start with any more, but check to be safe.
This fixes corruption of `docker pull` seen with rxvt-unicode (and likely other
terminals in that family) seen in #28111. Tested with rxvt-unicode
($TERM=rxvt-unicode), xterm ($TERM=xterm), mlterm ($TERM=mlterm) and aterm
($TERM=kterm).
The test cases have been updated to match the new behaviour.
Signed-off-by: Ian Campbell <ian.campbell@docker.com>
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