?2026 Synchronized Output

CSI ? 2026 h tells the terminal that a program is starting an update of the screen, and CSI ? 2026 l that it has finished. In between, the terminal keeps processing what it receives but does not draw it, so that the update appears all at once rather than half-done. A terminal that holds back drawing usually also gives up after a time limit, in case the program never resets the mode.

It is neither xterm’s nor DEC’s. Its definition is Synchronized Output, kept by the Contour terminal’s authors with their other VT extensions. It took iTerm2’s earlier synchronized updates and moved them to a DEC private mode, names the two halves BSU, begin synchronized update, and ESU, end synchronized update, and notes that there is no agreement yet on a time limit. A program finds out whether a terminal has it by asking with DECRQM: an answer of 0, not recognized, or no answer at all, means it does not.

A default xterm has no mode 2026. It ignores CSI ? 2026 h, and answers DECRQM for it with 0, as it does for any mode it does not know (do_dec_rqm). The cases follow xterm.

libghostty-vt implements the mode, and answers DECRQM with 1 or 2, so those cases are known differences. Holding back drawing is the job of whatever renders the screen, not of libghostty-vt’s terminal state, which goes on changing either way, as the second case shows.

Validation

SYNC-1: A default xterm does not recognize it

\e[?2026$p
\e[?2026h
\e[?2026$p

|____|
reply \e[?2026;0$y\e[?2026;0$y

SYNC-2: The screen still changes

\e[?2026h
AB
\e[?2026l

|AB__|
cursor 1,3