Hi Nicholas,
Thank you very much for your reply and for your interest in working on
this.
I should clarify that I am only a regular st user, rather than a
developer. My investigations led me to these observations about DECSET
2026 and the existing application-sync patch, but unfortunately I don't
have the skills or experience to implement the necessary changes myself.
That said, I would be very interested in following any work on this, and
I would be happy to help with testing if that could be useful. In
particular, I can test on my NetBSD system with st and lf and report any
issues or differences in behaviour.
If a patch or project takes shape, I would also appreciate being kept
informed of its progress, especially if it eventually results in a
solution that could address the issue on my system.
Thanks again for taking the time to look into this.
Best regards,
Stéphane
Le Fri, Aug 14, 2026 at 03:02:32AM +0300, Nicholas Bors-Sterian a écrit :
> On 12/8/26 2:25 am, Stéphane ORTEGA wrote:
>
> > Hello,
> >
> > I would like to ask about support for DEC private mode 2026 (Synchronized Output) in st.
> >
> > I am running NetBSD 11.0 amd64 with the current st master (commit 04ce0d6, 2026-06-29), built locally against NetBSD's X11R7. I use st as my terminal emulator and lf as my terminal file manager.
> >
> > While investigating a large number of error messages printed by st, I found that lf emits the following sequences when drawing its interface:
> >
> > CSI ? 2026 h
> > ..
> > CSI ? 2026 l
> >
> > A minimal reproduction is:
> >
> > printf '\033[?2026hHELLO\033[?2026l\n'
> >
> > This works correctly in xterm, but st reports:
> >
> > erresc: unknown private set/reset mode 2026
> >
> > The same messages are produced continuously when navigating in lf.
> >
> > I initially had the NetBSD/pkgsrc version of lf installed. I then removed it and built the latest upstream lf from GitHub with Go 1.26.5. The behaviour was unchanged, so this does not appear to be specific to the packaged version of lf.
> >
> > I also tested lf with an empty configuration, with the same result.
> >
> > I checked the existing synchronized-rendering support on the st website. The "sync" patch is described as having two parts:
> >
> > auto-sync, which has already been merged upstream;
> >
> > application-sync, which is available as the historical st-appsync patch.
> >
> > However, the application-sync patch appears to implement the older DCS protocol:
> >
> > ESC P = 1 s ESC \
> > ESC P = 2 s ESC \
> >
> > rather than the DECSET/DECRST 2026 sequences:
> >
> > CSI ? 2026 h
> > CSI ? 2026 l
> >
> > The existing st application-sync patch therefore does not solve this particular case.
> >
> > I also confirmed that the current st master does not handle private mode 2026: it reaches the "unknown private set/reset mode" error path in tsetmode().
> >
> > My questions are:
> >
> > Is there currently any work or patch for supporting DECSET/DECRST 2026 in st?
> >
> > Is the older application-sync implementation intentionally considered sufficient, or is support for the newer CSI ?2026h/l mechanism desirable?
> >
> > If support for DEC mode 2026 is considered appropriate for st, would a patch implementing synchronized output at the rendering level be acceptable?
> >
> > I am particularly interested in the distinction between merely accepting mode 2026 (and suppressing the error) and actually implementing its semantics: while synchronized output is active, terminal input should still be processed, but screen updates should not be presented until the corresponding 2026l is received, with an appropriate timeout as a safeguard.
> >
> > For reference, my minimal reproduction is simply:
> >
> > printf '\033[?2026hHELLO\033[?2026l\n'
> >
> > With xterm, this produces "HELLO" without an error. With current st master, it produces the "unknown private set/reset mode 2026" diagnostic.
> >
> > I would appreciate any pointers to existing work, design considerations, or objections before attempting an implementation for st.
> >
> > Thanks.
> Hey Stéphane,
>
> I would be interested in working on this -- please send me a patch if you
> have already started work on one.
>
> I think this is something that st should support by default, however the
> existing application-sync patch you mention being a community patch that's
> not seeing inclusion in the mainline leads me to believe that the
> maintainers want to avoid supporting too many 'niche' escape code sequences.
> Perhaps an update to (or derivative of) the existing sync patch would be
> most suitable?
>
> As for your second question, I think that a good implementation should
> implement the expected semantics (with the safeguard timeout as you
> mentioned) as opposed to silently ignoring the sequence would be best.
>
> Thanks for looking into this, I'll be happy to look at any code you post.
--
Stéphane ORTEGA https://stephaneortega.fr/ +33 (0)684.041.350
GPG Key ID: 0x56F6CF155AB3B3BF/2048R IRC: Well_AT_{libera,oftc}
Received on Sat Aug 15 2026 - 22:47:54 CEST