vt plumbing: empty selection

[ori@eigenstate.org]

Plumbing text in vt requires selecting the text that you
want to plumb precisely. This patch makes plumbing behave
the same way that it does in rio, though cwd isn't updated
whenever we change directories. That'll take further thought
to get right.

There's an escape code (OSC 7) for programs to advertise
their current working directory to the terminal emulator,
and I'm willing to implement it --  but nothing on 9front
generates it, and it's not clear that we want to start.
The programs that do generate it are likely to be on the
other end of an ssh connection, which means that their cwd
is unlikely to make sense to plumb. Is there anything that
is useful to do here?

diff -r 3bcb5998f222 sys/src/cmd/vt/main.c
--- a/sys/src/cmd/vt/main.c	Wed Apr 08 23:48:09 2020 +0200
+++ b/sys/src/cmd/vt/main.c	Wed Apr 08 21:07:46 2020 -0700
@@ -170,7 +170,7 @@
 void	escapedump(int,uchar *,int);
 void	paste(void);
 void	snarfsel(void);
-void	plumbsel(void);
+void	plumbsel(Point);
 
 static Channel *pidchan;
 
@@ -982,13 +982,47 @@
 	free(s);
 }
 
+/*
+ * Grabs the non-whitespace text around a character
+ * cell, matching the behavior in rio for plumbing.
+ * Does not modify the selection.
+ */
+char*
+surrounding(Point p)
+{
+	int c, x0, x1;
+	char *s, *e;
+
+	for(x0 = p.x; x0 > 0; x0--){
+		c = *onscreenr(x0 - 1, p.y);
+		if(c == 0 || c == ' ' || c == '\t' || c == '\n')
+			break;
+	}
+	for(x1 = p.x; x1 <= xmax; x1++){
+		c = *onscreenr(x1 + 1, p.y);
+		if(c == 0 || c == ' ' || c == '\t' || c == '\n')
+			break;
+	}
+	s = malloc((x1 - x0 + 1)*UTFmax);
+	if(s == nil || x0 == x1)
+		return nil;
+	e = selrange(s, x0, p.y, x1, p.y);
+	*e = 0;
+	for(e = s; *e; e++)
+		print("%c(%d)\n", *e, *e);
+	return s;
+}
+
 void
-plumbsel(void)
+plumbsel(Point p)
 {
 	char *s, wdir[1024];
 	int plumb;
 
-	if((s = selection()) == nil)
+	s = selection();
+	if(s == nil || *s == 0)
+		s = surrounding(p);
+	if(s == nil)
 		return;
 	if(getwd(wdir, sizeof wdir) == nil){
 		free(s);
@@ -1116,6 +1150,9 @@
 void
 readmenu(void)
 {
+	Point p;
+
+	p = pos(mc->xy);
 	if(button3()) {
 		menu3.item[1] = ttystate[cs->raw].crnl ? "cr" : "crnl";
 		menu3.item[2] = ttystate[cs->raw].nlcr ? "nl" : "nlcr";
@@ -1173,7 +1210,7 @@
 		return;
 
 	case Mplumb:
-		plumbsel();
+		plumbsel(p);
 		return;
 
 	case Mpage:		/* pause and clear at end of screen */