?2028 Text Reflow

Most terminals now reflow when they are resized: a line that wrapped is rewrapped to the new width, rather than cut off when the window narrows or left broken when it widens. That suits a shell’s output, but not a program that lays out its own lines and would rather they stayed where it put them. CSI ? 2028 l turns reflow off for the line the cursor is on and the lines below it, and CSI ? 2028 h turns it back on.

It is neither xterm’s nor DEC’s. Its definition is Line Reflow Reconfiguration, kept by the Contour terminal’s authors with their other VT extensions, read at commit 5a6be84. That is the whole of it: the two settings, and DECRQM to find out whether a terminal has the mode. It does not say what the mode is to begin with, what “the following lines” are, or what a line keeps when the mode changes after it was written.

Contour’s own implementation fills some of that in (Terminal::setMode). The mode begins as Contour’s allowReflowOnResize setting, which is on by default, and DECSTR and RIS put it back there (Terminal.cpp, Terminal.cpp). Reflow is a property of each line rather than of the screen. Resetting the mode marks the cursor’s line and every line below it on the page as not to be rewrapped, but setting it marks every line of the page, above the cursor too. Either way only the main screen is affected, and with allowReflowOnResize off the mode can be changed but changes nothing.

A default xterm has no mode 2028, and does not reflow at all. It answers DECRQM for the mode with 0, as it does for any mode it does not know (do_dec_rqm). The cases follow xterm. A case cannot resize the screen, so they cannot show reflow either way; they check what DECRQM says and that the mode changes nothing on the screen.

libghostty-vt does not recognize mode 2028 either, and answers DECRQM for it with 0. When it is resized it reflows the main screen if DECAWM is set, and never the alternate screen, and it has no way to turn reflow off for some lines and not others.

Support

Validation

REFLOW-1: A default xterm does not recognize it

\e[?2028$p
\e[?2028l
\e[?2028$p

|____|
reply \e[?2028;0$y\e[?2028;0$y

REFLOW-2: The screen does not change

AB
\e[?2028l
CD
\e[?2028h

|ABCD|
|____|
cursor 1,4
pending-wrap yes