Terminate only OSC strings with BEL

DCS and APC end with ST. Stopping at a BEL in their payload framed just
the first half and left the rest to be typed into the query.

Reported by Copilot on #4926.

src/tui/light.go

@@ -363,13 +363,21 @@ func csiEnd(buffer []byte) int {
 }
 
 // stringEnd returns the length of the string sequence (DCS, OSC or APC) at the
-// start of the buffer, or 0 if its terminator has not arrived. The terminator is
-// ST, and BEL as well because xterm has always allowed it.
+// start of the buffer, or 0 if its terminator has not arrived.
 func stringEnd(buffer []byte) int {
+	if len(buffer) < 2 {
+		return 0
+	}
+	// Every one of them ends with ST. BEL ends an OSC as well, because xterm has
+	// always allowed it, but stopping at a BEL inside a DCS or APC payload would
+	// frame only its first half and leave the rest to be typed into the query.
+	bel := buffer[1] == ']'
 	for i := 2; i < len(buffer); i++ {
 		switch buffer[i] {
 		case '\a':
-			return i + 1
+			if bel {
+				return i + 1
+			}
 		case Esc.Byte():
 			if i+1 < len(buffer) && buffer[i+1] == '\\' {
 				return i + 2

src/tui/light_csi_test.go

@@ -28,7 +28,8 @@ func TestUnknownCSISequence(t *testing.T) {
 		{"\x1b]11;rgb:4a4a/4a4a/4a4a\x1b\\", Invalid, 25}, // background color reply
 		{"\x1b]0;a title\a", Invalid, 12},                 // BEL terminated
 		{"\x1bP>|kitty(0.48.2)\x1b\\", Invalid, 19},       // XTVERSION reply
-		{"\x1b_Gi=1;OK\x1b\\", Invalid, 11},               // kitty graphics reply
+		{"\x1b_Gi=1;OK\x1b\\", Invalid, 11},
+		{"\x1bP\ax\x1b\\", Invalid, 6}, // BEL inside a DCS payload is not a terminator               // kitty graphics reply
 
 		// Left alone: this is how ALT-[ and ALT-] arrive
 		{"\x1b[a", Alt, 2},
@@ -71,6 +72,9 @@ func TestStringEnd(t *testing.T) {
 		want   int
 	}{
 		{"\x1b]0;t\a", 6},
+		{"\x1bP\ax\x1b\\", 6}, // BEL in the payload ignored, ST ends it
+		{"\x1bP\a", 0},        // BEL does not terminate a DCS
+		{"\x1b_Gi=1\a", 0},    // nor an APC
 		{"\x1b]0;t\x1b\\", 7},
 		{"\x1b_G\x1b\\", 5},
 		{"\x1b]0;t", 0},     // no terminator