Terminal emulators typically receive window resize events by installing a signal handler for the SIGWINCH signal. Handling of these signals can create challenges due to their inherently racy properties. Resize events must be synchronized with other application state in a safe manner.
Standard control sequences exist to query the current terminal size from the
terminal and receive an in-band control sequence with the window size. However,
this system requires polling - which is not ideal. Usually, SIGWINCH handling
is preferred.
This specification defines a single new Private Mode which enables automatic reporting of in-band text area resize events -
2048- Enable/disable reports for text area size in both characters and pixels
Note
When using ReadConsoleInput on Windows, resize notifications are already
delivered in band.
Detection is performed with a standard DECRQM query:
CSI ? 2048 $ p
To which the terminal will respond with a DECRPM response:
CSI ? 2048 ; Ps $ y
A Ps value of 0 or 4 means the mode is not supported. Reference
The reports can be enabled using DECSET or DECRST control sequences:
CSI ? 2048 h to enable the mode.
CSI ? 2048 l to disable the mode.
The format of the response / notification is:
CSI 48 ; height_chars ; width_chars ; height_pix ; width_pix t
If a terminal is not capable of reporting pixel sizes, it must report them as 0. A terminal MUST report pixel sizes if it is capable of reporting them.
Any field MAY contain sub-parameters, separated by colons (':', ASCII 0x3A). If a client does not understand these, it MUST ignore subparameters. Currently, no subparameters are defined.
Note
The reported size MUST be the text area size. Text area does not include any padding the terminal applies to the window
When first enabled, the terminal MUST send a report of the current size.
If the mode is already enabled, the terminal MUST immediately report the current size if an attempt is made to enable the feature.
This specification does not dictate any sort of throttling or limiting the frequency with which reports can be sent.
The terminal MUST NOT send notifications until the internal resize is complete. That is, the terminal must be prepared for the TTY and application to behave at the new size prior to sending the sequence.
Important
The reported area MUST be the text area size. Font size changes can also affect the text area size
An application queries the terminal for support:
A: \x1b[?2048$p
The terminal responds with:
T: \x1b[?2048;2y
The mode is supported but not currently enabled. The application enables the mode.
A: \x1b[?2048h
The terminal turns the mode on, and gives an immediate report of the window size.
T: \x1b[48;24;80;240;1600t
After some time, the user changes the window size. The terminal sends a new size report.
T: \x1b[48;48;80;480;1600t
By my analysis the following 33 terminals tested, 4 support in-band resize notification, https://ucs-detect.readthedocs.io/results.html#dec-private-modes-support
And I have implemented it in my python TUI library, blessed, documented here https://blessed.readthedocs.io/en/latest/measuring.html#resizing
Because I feel strongly that this is needed, because SIGWINCH, and, its carrying protocol features/options (telnet, ssh) sometimes cannot reliably transmit about window size changes, I wrote previously in this demonstration program https://github.com/jquast/blessed/blob/master/bin/resize.py