When using tmux I need to set ttymouse to xterm2 or sgr (preferred, since it supports more columns) to drag window borders. ¹

I was wondering whether it’d be a good idea to make sgr the default for tmux. There was a PR/issue for changing the default for screen:

• Set ttymouse to "sgr" for screen #4146
• [WIP] Detect SGR mouse support for screen #5042

The issue mentions that tmux supports SGR since 2013 (1.8) and that its DA2 code is 84, but the PR only changed the default of screen. I’ve naïvely added the same simple detection to term.c and it worked for me:

diff --git i/src/term.c w/src/term.c
index b789f3924..c9a2b4126 100644
--- i/src/term.c
+++ w/src/term.c
@@ -5220,6 +5220,12 @@ handle_version_response(int first, int *arg, int argc, char_u *tp)
 		term_props[TPR_MOUSE].tpr_status = TPR_MOUSE_XTERM;
 	}

+	if (arg[0] == 84)
+	{
+	 	// tmux supports SGR since basically forever
+		term_props[TPR_MOUSE].tpr_status = TPR_MOUSE_SGR;
+	}
+
 	// If no recognized terminal has set mouse behavior, assume xterm.
 	if (term_props[TPR_MOUSE].tpr_status == TPR_UNKNOWN)
 	{

But I’m not sure whether this is a good change for everyone, because of the following questions:

Checking terminal name vs device attributes

In os_unix.c there is also a check for the terminal name (not DA2 response) being tmux which effectively results in the current default of ttymouse=xterm.

From my test I know that the terminal capabilities will take precedence over the name, but is it a good approach? It would mean that someone setting $TERM=tmux without actually being on tmux would not get the new default (might be good or bad).

The resulting documentation might also get very confusing.

Is sgr/xterm2 a sensible default?

Is there a constellation where xterm2 instead of xterm could cause problems?
(sgr seems to be irrelevant here, since it falls back to xterm2 behaviour according to docs when no SGR sequences are sent, and tmux also falls back to X10 sequences if the underlying terminal does not send SGR sequences)

I see two places where xterm2 differs from xterm, and I don’t fully understand the consequences: