DECELR, DECSLE, DECRQLP, DECEFR, DECLKD Locator

DEC calls a mouse or a tablet a locator. With these functions a host can ask a terminal that has one to report where it is and which buttons are down:

A report is DECLRP, CSI Pe ; Pb ; Pr ; Pc ; Pp & w: the event, the buttons down, then the row, column and page of the locator.

All of this is DEC STD 070’s Text Locator Extension, section 13, an optional extension (its levels read “1x, 2x, 3x, 4x”). DECELR, DECSLE, DECRQLP and DECEFR are in its chapter 4, from p. 5, with DECLRP on p. 6; DECLKD, for the separate Locator Port Extension, is §5.4 on p. 13. A terminal that has the extension always answers a request, so an application never waits for a report that will not come. With locator reports disabled, DECRQLP is answered with event code 0, “the locator is unavailable”, and no other parameters, CSI 0 & w (p. 8), and so is a DECEFR received while reports are disabled.

A default xterm does not have the extension. It is built in only with --enable-dec-locator, which is off unless asked for (configure.in), and without it xterm’s parser maps the final byte of all four CSI sequences to the handling it gives a malformed one (VTPrsTbl.c). That does not end the sequence: it goes on ignoring until the next byte from @ to ~, and takes that as the end (charproc.c), so the first letter or the like after the sequence is swallowed. That is a bug, but this site follows a default xterm, bugs included. DECLKD is not implemented in any build: xterm’s DCS handling acts on $ only when t follows it (misc.c), and its graphics code lists “locator key definitions (DECLKD)” as still to do (graphics.c). The cases follow a default xterm, as the sources page explains, so they expect these sequences to draw no report, and to swallow the letter after them. That is what DEC STD 070 asks of a terminal without the extension, too, since it says a terminal ignores a function it does not implement (§3.5.1.3, p. 3-20); a terminal with it would answer DECRQLP. libghostty-vt has no handler for any of them, and ignores them, but ends each at its final byte, so the letter after it is printed; those cases are known differences.

xterm and libghostty-vt both have a different kind of mouse reporting, xterm’s own, mouse tracking, turned on with private modes such as CSI ? 1000 h. The locator’s own status request, CSI ? 55 n, is on the DEC status reports page.

Support

Validation

DECELR-1: The letter after it is swallowed

A
\e[1;2'z  # DECELR: reports on, in character cells
B         # taken as the end of the ignored sequence

|A_____|
cursor 1,2
reply none

DECELR-2: No locator, no report

\e[1;2'z  # DECELR
\e['|     # DECRQLP

|______|
reply none

DECELR-3: DECSLE and DECEFR are consumed, and so is the letter after

A
\e[1;3'{  # DECSLE: report presses and releases
\e[1;1;2;2'w # DECEFR
B

|A_____|
reply none

DECELR-4: DECLKD’s string is consumed

A
\eP0$w1/41/42\e\\ # DECLKD: button 1 sends A down, B up
B

|AB____|
reply none