Wait for a string terminator split across reads

A trailing ESC was read as a lone ESC, so the read loop stopped waiting and
the parser was handed an unterminated sequence, whose payload was typed into
the query. With ESCDELAY=0 nothing covered it.

Reported by Copilot on #4926.

src/tui/light.go

@@ -414,6 +414,14 @@ func incompleteEscape(buffer []byte) bool {
 		tail = tail[len(tail)-escapeLookback:]
 	}
 	start := bytes.LastIndexByte(tail, Esc.Byte())
+	// A trailing ESC can be the first half of a string terminator. Reading it as
+	// a lone ESC ends the wait and hands the parser an unterminated sequence, so
+	// fall back to the one this ESC would have terminated.
+	if start == len(tail)-1 && start > 0 {
+		if prev := bytes.LastIndexByte(tail[:start], Esc.Byte()); prev >= 0 {
+			start = prev
+		}
+	}
 	if start < 0 || len(tail)-start < 2 {
 		return false
 	}

src/tui/light_escape_test.go

@@ -65,6 +65,18 @@ func TestIncompleteStringEscape(t *testing.T) {
 		{"\x1b]0;title\x1b\\", false},
 		{"\x1bP>|kitty(0.48.2)\x1b\\", false},
 		{"ab\x1b]11;rgb:", true},
+
+		// A string terminator split across reads: the ESC has arrived, the
+		// backslash has not
+		{"\x1b]0;t\x1b", true},
+		{"\x1bP>|kitty\x1b", true},
+		{"\x1b_G\x1b", true},
+
+		// Complete sequence followed by a lone ESC, which is the ESC key
+		{"\x1b]0;title\a\x1b", false},
+		{"\x1b\x1b", false},
+		{"\x1ba\x1b", false},
+		{"\x1b[A\x1b", false},
 	} {
 		if got := incompleteEscape([]byte(c.buffer)); got != c.want {
 			t.Errorf("incompleteEscape(%q) = %v, want %v", c.buffer, got, c.want)