bin/59977: sh(1): input line editor sometimes blanks part of the input

["campbell+netbsd%mumble.net@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         59977
>Category:       bin
>Synopsis:       sh(1): input line editor sometimes blanks part of the input
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Feb 08 01:25:00 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, 9, ...
>Organization:
>Environment:
>Description:
Sometimes, when editing the middle of a line in sh(1), part of the rest of the line is blanked.  The text is still there under if I continue editing or redraw the screen, but it is displayed as blank.
>How-To-Repeat:
1. Type the following into the shell:

   $ quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland

2. Move the cursor to the start of the second `quagga' (`|' marks the cursor):

   $ quagga eland |quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland

3. Type `M-d' to delete that quagga:

   $ quagga eland | eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga eland

4. Type `quagga' to replace it -- as soon as I enter the second `a' in `quagga'; the input line reliably turns into:

   $ quagga eland quagga eland quagga eland quagga eland quagga eland quagga _eland quagga eland quagga eland quagga eland quagga eland quagga eland quagga_ eland quagga eland quagga eland quagga eland quagga eland quagga eland

   where the underscored part is displayed as blank in my terminal, but the text is still there under the hood, e.g. if I redraw the screen with C-l.
>Fix:
Yes, please!  This has been a problem as long as I can remember using NetBSD and it's really annoying!  I'm not sure whether it's an sh(1) bug, a libedit(3) bug, a kernel tty subsystem bug, or what.