DECCOLM Column Mode

CSI ? 3 h makes the screen 132 columns wide, and CSI ? 3 l makes it 80. Either way, DEC STD 070 says it clears the screen, removes the scrolling region, and moves the cursor to the top-left, “even if the terminal was already in the selected state”. Every line goes back to single-width too. DEC STD 070 makes column mode an optional extension at every level, marked 1X to 4X.

xterm only does any of this when it has been allowed to, with CSI ? 40 h, xterm’s allow 80/132 column mode; without it, DECCOLM is ignored. Mode 40 is off by default (the c132 resource is False). With it on, xterm clears the screen, asks for the new width, then resets the line sizes and margins and homes the cursor (srm_DECCOLM, set_column_mode). Mode 95, DECNCSM, can stop it clearing the screen; it is not covered here.

The case runner draws only as many columns as each expected screen has, but a cursor sent to column 999 stops at the real last column, which shows the width. DECRQM reports both modes.

libghostty-vt implements DECCOLM and mode 40 as xterm does.

Validation

DECCOLM-1: Ignored unless allowed

ABC
\e[2;3H
\e[?3h    # 132 columns, not allowed
\e[?3$p   # ask about column mode

|ABC___|
|______|
cursor 2,3
reply \e[?3;2$y

DECCOLM-2: Mode 40 is off by default

\e[?40$p
\e[?40h
\e[?40$p

|______|
reply \e[?40;2$y\e[?40;1$y

DECCOLM-3: 132 columns, cleared, cursor home

\e[?40h   # allow 80/132
ABC
\e[2;3H
\e[?3h    # 132 columns

|______|
|______|
cursor 1,1

DECCOLM-4: The new width

\e[?40h   # allow 80/132
\e[?3h    # 132 columns
\e[1;999H
\e[?3$p

|______|
cursor 1,132
reply \e[?3;1$y

DECCOLM-5: Back to 80

\e[?40h   # allow 80/132
\e[?3h    # 132 columns
\e[?3l    # 80 columns
\e[1;999H

|______|
cursor 1,80

DECCOLM-6: The margins are removed

\e[?40h   # allow 80/132
\e[1;2r   # region is lines 1 and 2
\e[?69h   # allow left and right margins
\e[1;2s   # margins at columns 1 and 2
\e[?3h    # 132 columns
\r\nHello # would wrap at column 2 and stay in lines 1 and 2
\r\nWorld

|______|
|Hello_|
|World_|
|______|