xsrc/56279: line editing performance regression in xterm(?)

[dholland%NetBSD.org@localhost]

>Number:         56279
>Category:       xsrc
>Synopsis:       line editing performance regression in xterm(?)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    xsrc-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jun 27 17:20:00 +0000 2021
>Originator:     David A. Holland
>Release:        NetBSD 9.99.85 (20210623)
>Organization:
>Environment:
System: NetBSD valkyrie 9.99.85 NetBSD 9.99.85 (VALKYRIE) #7: Wed Jun 23 18:32:25 EDT 2021  dholland@valkyrie:/usr/src/sys/arch/amd64/compile/VALKYRIE amd64
Architecture: x86_64
Machine: amd64
>Description:

After updating to 9.99.85 line editing performance in xterm,
specifically of inserting characters at the beginning of a long line,
seems to have become awful.

If you paste a few lines' worth of junk, go to the beginning of the
input, and start typing, it's now easy to type faster than the display
updating can keep up with. At least in xterm. I first noticed this in
tcsh, but it also affects sh, though curiously it seems to _not_
affect gdb from base.

Don't know when it started since I hadn't updated in a while, but it
wasn't like this a year ago.

It doesn't happen on the console so it's _probably_ something in xterm
but that's not entirely clear either.

It happened both with a tcsh bin from before updating and also after
recompiling.

>How-To-Repeat:

as above

>Fix:
Dunno.